( PDF ) A Laboratory Course in Physiological Psychology

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A LABORATORY COURSE IN PHYSIOLOGICAL

PSYCHOLOGY.

By Edmund C Sanford (1891-1893)·

Classics in the History of Psychology

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Christopher D. Green

York University, Toronto, Ontario

ISSN 1492-3173

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A LABORATORY COURSE IN PHYSIOLOGICAL PSYCHOLOGY.

By Edmund C Sanford (1891-1893)·

First published in American Journal of Psychology, 4, 141-155, 303-322, 474-490; 5, 390-?,

593-?.

Posted June 2000

First Paper (I. Dermal Senses, II. Static and Kinæsthetic Senses)

Second Paper (III. Taste and Smell, IV. Hearing)

Third Paper (V. Vision)

Fourth Paper (V. Vision cont'd)

[Classics Editor's note: There are widespread inconsistencies in the use of punctuation,

abbreviation, italics, and even spelling in the original publication of Sanford's "Course." These

idiosyncrasies have been reproduced here as accurately as possible. The use of "[sic]" has

been confined only to instances that might otherwise lead to confusion.]

[First Paper, 1891, pp. 141-155]

After Prof. Ladd's careful statement or the psycho-physiological facts and Prof. James's brilliant

exposition of their psychological and even metaphysical import, it is no longer necessary to

argue the importance of the subject matter of this branch of the new psychology. No one that

has once seen the new is going to be satisfied any longer with the old. But the appropriation of

new facts alone is not sufficient to elevate psychology to its true place in the circle of sciences.

As long as psychologists live upon the crumbs that fall from the tables of neurology and

physiology they will live in dependence. They must investigate for themselves, -- no less

rigorously and no less broad-mindedly than others, but from their own standpoint, and must

view what they find in its psychological perspective. This means that a prominent place must be

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given to psychological laboratories for research; and the friends of psychology already

congratulate themselves on the beginning of several of great promise in this country.

Beyond this, however, lies another thing of cardinal importance, namely, the adoption of a right

pedagogical method. The student of psychology must have its facts and principles brought

home to him in a way not inferior to the best in other sciences, if psychology is to have the

infusion of new vigor that they have had, and afford the healthy and virile training that they

afford. He must see for himself the phenomena about which he psychologizes, he must perform

experiments, he must have the inside view. The new psychology has been said to do away with

introspection, but that is a mistake. It retains introspection [p. 142] and refines and gives it

precision by making it operate under experimental conditions; and it is just these inner aspects

that are particularly hard for the student to frame for himself from bare descriptions. He must

himself serve as subject of the experiment before he can really understand it. To say, as has

recently been said, that a few models or the brain and a color-mixer are about all the apparatus

needed for a course in physiological psychology savors of the scholasticism from which we

hope to have escaped. Notwithstanding its better material, such a method must lead to the

same text-book work and the same artificial general conceptions as of old. For those especially

that are to work in any of the fields of applied psychology, in pedagogy, or criminology, or even

theology, the intimate laboratory knowledge (and its parallels in anthropological and

comparative psychology) is essential to an effective grasp of their subjects. The need of such

an apprenticeship for later work in the research laboratory is of course obvious. That such a

course is even now desired by open eyed teachers is shown by the inquiries made for it of

those known to be engaged in experimental psychological work.

Just what experiments such a course should contain is itself as yet a matter of experiment; but

that it should, if it aims at any thing like proportion, introduce the student to all the chief

methods of research and cause him to observe for himself all the more important phenomena

seems reasonable. Such a course has been in mind in the collection of the experiments which

is begun below, and which is to be continued in successive numbers or the JOURNAL till

completed. That the list is complete or the selection always the best the author is very far from

maintaining -- to mention a large omission only no experiments on hypnotism are now

proposed, because they seem unfitted to beginners in the field. And in any event the ideal

laboratory course can only be reached after repeated adaptation and long trial in actual use.

This course had its origin in a series of notes which it was found necessary to make for the use

of a group of students taking my practice course during the past year. The experiments have

been performed in the laboratory here, and all, except those added in this revision, by the

students themselves. The demonstrational character of the work has been kept in mind, and

the experiments chosen are generally rather qualitative than quantitative, even where for

convenience they have been given a quantitative form. In selecting apparatus the simplest that

promised the desired result has generally been chosen; and while this makes the course by no

means representative of the facilities of this laboratory much less of the possibilities of

psychological experimentation, it may perhaps make it useful to those teachers -- unfortunately

too many -- whose equipment must be brought within the compass of a scanty appropriation. A

large part of the absolutely essential apparatus could be made by the teacher himself, and

almost all, I doubt not, with the assistance of common mechanics. The notes on apparatus and

references to literature that are inserted from time to time will open the way to more elaborate

experiments and apparatus for those that desire them.

I. -- THE DERMAL SENSES.

SENSATIONS OF CONTACT.

Apparatus. The experiments on the Sense of Locality require no special apparatus. Those on

Discriminative Sensibility can be made with ordinary drawing dividers; but if these are used, it

will be well to stick the points into little pointed tips of cork to avoid the sharpness and coldness

of the metal. (An excellent, but more expensive, Æsthesiometer is made by C. Verdin, 7 Rue

Linne, Paris, at 35 francs; for the description of an elaborate and very convenient one, see

AMER. JOUR. [p. 143] PSYCHOL. I, 552.) Something is also needed in experiment 6 d for

ads:

rendering the skin anæsthesic.

1. The Sense of Locality. Touch your skin several places with the same object, and analyze

out, as far as you can, the particular quality of the sensation by which you recognize the place

touched. This quality of the sensation is known as the "Local Sign."

2. Cause the subject to close his eyes; touch him on the fore-arm with a pencil point; and

require him to touch the same point with another pencil immediately afterward. Estimate the

error in millimeters and average the results for a number of trials, noting the direction of error; if

it is constant. The subject may be allowed to correct his placing of the pencil if not satisfied with

it on first contact.

3. Aristotle's Experiment. Cross the middle finger over the first in such a way as to bring the tip

of the middle finger on the thumb side of the first finger. Insert between the two a pea or other

small object. A more or less distinct sensation, of two objects will result, especially when the

fingers are moved.

4. Judgments of Motion on the Skin. a. Subject with closed eyes. Resting a. pencil point or the

head of a pin gently on the fore-arm, move it slowly and evenly up or down the arm. Require

the subject to indicate his earliest judgment of the direction. Ii the experiment is carefully made,

the fact of motion will be perceived before its direction, b. Try a number of times, estimating the

distances traversed in millimeter averaging for the two directions separately. It will probably be

found that the downward distances have been greater than the upward. c. Starting from a fixed

point on the fore-arm move the pencil in irregular order up, down, right or left, and require the

subject to announce the direction of motion as before.

Cf. Hall and Donaldson, Motor Sensations of the Skin; Mind, X, 1885, 557.

5. Rest the fingers lightly on the forehead and move the head from side to side keeping the

fingers motionless. Almost the whole of the motion will be attributed to the fingers. Light tapping

of the forehead with the finger we feel in the forehead more markedly than in the finger. With

our own hand on our forehead we feel the forehead; some one else's hand we feel the hand.

6. Weber's Sensory Circles. a. Find the least distance apart a which the points of the

æsthesiometric compasses can be recognized as two when applied to the skin of the fore-arm.

Try also the upper arm, the back of the hand, the forehead, the finger-tip and the tip of the

tongue. Be very careful to put both points on the skin at the same time and to bear on equally

with both. b. Compare the distance between the points just recognizable as two when applied

lengthwise of the arm with that found when they are applied cross-wise. c. Give the points a

slightly less separation than that found for the fore-arm (crosswise) and beginning at the elbow

draw the points downward side by side along the arm. They will at first appear as one, later as

two, after which they will appear to separate as they descend. Something similar will be found

on drawing the points from side to side across the face so that one shall go above, the other

below the mouth. d. Make the skin anæsthesic with an ether spray and test the discriminative

sensibility as before.

Cf. Weber's measurements as given in the text-books.

7. Filled space is relatively under-estimated by the skin. Set up in small wooden rod a row of

five pins separated by intervals of half an inch, and in another two pins an inch and a half apart.

Apply to the arm like the compasses above. The space occupied by the five pins will seem less

than that between the two. [p. 144]

8. Active touch, that is touch with movement, is far more discriminating than mere contact.

Compare the sensations received from resting the tip or the finger on a rough covered book

with those received when the finger is moved and the surface "felt of."

9. The time discriminations or the sense of contact are very delicate. Strike a tuning-fork, touch

it near the bottom or the prong and immediately remove the finger so as not to stop the fork.

The taps of the fork on the skin do not blend into a continuous sensation for the tactual sense,

even when the vibrations are 1000 or more a second. For sensations of minimal contact, see

Ex. 22.

On the foregoing experiments, cf. Weber, Tastsinn und Gemeingefühl, Wagner's

Handwörterbuch der Physiologie, Vol. III. pt. 2; Funke, Hermann's Handbuch der Physiologie,

Vol. III, pt. 2.

SENSATIONS OF TEMPERATURE.

Apparatus. Two brass rods (6 inches long and 0.25 inch in diameter, turned down to a fine

smooth point 0.5 mm. in diameter), paper ruled in mm. squares, menthol pencil (such as is

used for headaches), centigrade thermometers, vessels of water at different temperatures.

10. Hot and Cold Spots. a. Move one of the pointed brass rods, or even a cool lead pencil

slowly and lightly over the skin of the back or the hand. At certain points distinct sensations of

cold will dash out, while at others no temperature sensation will be perceived, or at meet, only a

faint and diffuse one. Heat one or the rode and repeat the experiment. b. On some convenient

portion of the skin mark off the corners of a square 2 cm. on the side. Go over this square

carefully both lengthwise and crosswise for both heat and cold, drawing the point along lines 1

mm. apart, and note on a corresponding square of millimeter paper the hot and cold spots

found, hot spots with red ink, cold with black. This time the points should be heated or cooled

considerably by placing them in vessels of hot or cold water and should be kept at an

approximately constant temperature by frequent change, one being left in the water while the

other is in use. Break the experiment into a number of sittings so se to avoid fatiguing the

spots; for they are very readily fatigued. A map made in this way cannot hope to represent all

the spots, but It will suffice to show the permanence of some of them and possibly to show their

general arrangement. c. Notice the very distinct persistence of the sensations after the point

has been removed.

11. The temperature spots respond with their characteristic sensations to mechanical (and

electrical) stimulation, and do not give pain when punctured. a. Choose a very certainly located

cold spot and tap it gently with a fine wooden point (not too soon after locating it, if it has been

fatigued in locating); or better have an assistant tap it. b. Thrust a needle into a well located

cold point. Try both for comparison on an adjacent portion of the skin.

12. The temperature spots respond to chemical stimulation. Choose a convenient area, say on

the back of the hand, and take its temperature carefully, allowing the thermometer to remain in

contact with the skin as long as it continues to rise. Note the temperature and rub the skin

lightly with a menthol pencil. After a little the sensation of cold will appear. Take the

temperature of the skin again; it will be found as high or higher than before, in spite of the

contrary sensation. The menthol makes the nerves of cold at first hyperæsthesic (so that they

respond with their special sensation to mere contact, and give an intenser sensation when a

cold body is applied than do adjacent normal portions of the skin); afterward, however, all the

cutaneous nerves become more or less anæsthestc. [p. 145]

Cf. on the foregoing experiments: Blix, Zeitschrift für Biologie, Bd. XX, H. 2. 1884.

Goldscheider. Neue Thatsachen über die Hautsinnesnerven, Du Bois-Reymond's Archiv,

Supplement-Band, 1885 pp. 1-110; Donaldson, On the Temperature-sense. Mind. X. 1885; and

the literature cited by these authors. On the chemical stimulation of the temperature nerves:

(Cold) Goldscheider Ueber die specifische Wirkung des Menthols auf die Temperatur-Nerven.

Verh. d. Berliner physiol. Gesell. 9 April, 1886, Du Bois-Reymond's Archiv, 1886, p. 555. (heat)

Die einwirkung der Kohlensäure auf die sensiblen Nerven des Haut, Verh. d. Berliner physiol.

Gesell. 25 Nov. 1887, Du Bois-Reymond's Achiv, 1888.

13. The temperature of the skin at any moment is a balance between its gain and loss or heat.

Anything that disturbs that balance, causing increased gain or loss of heat, produces

temperature sensations. It is common experience that a piece of cloth, a bit of wood, a piece or

metal, all of the same temperature as the air that seems indifferent to the hand, cause different

degrees of the sensation or cold when touched, because they increase the lose of heat by

conduction in different degrees. If a paper bag be placed over the hand held upward, a

sensation of warmth is soon felt, because or the decreased loss of heat.

14. Provide three vessels of water one at 30°c., the second at 40°, the third at 20°. Put a finger

or one hand into the warmer water, a finger of the other into the cooler. At first the usual

temperature sensations will be felt, but after a little they disappear more or less completely,

because of the fatigue of the corresponding temperature organs. Now transfer both fingers to

the water of normal temperature. It will seem cool to the finger from warmer water and warm to

the one from cooler.

15. The intensity of the temperature sensation depends on the amount of surface stimulated.

Dip a finger in cold water, then the whole hand. Notice the increase in sensation.

16. The fatigue of the temperature apparatus may produce an apparent contradiction of Ex. 15.

Dip one hand entirely under cold water and keep it there for a moment. Then dip the finger or

the other hand or the whole hand several times in the same water, withdrawing it immediately

each time. The water seems colder to the finger or hand which is only dipped.

17. Hold a very cold piece of metal on the forehead or on the palm of the hand for hall a minute.

On removing it the sensation of cold continues though the actual temperature of the skin is

rising. Sometimes fluctuations are observed in the persisting sensation. After contact with a hot

body the sensation of heat continues in the same way, though the temperature of the skin falls.

Goldscheider explains this result for cold in part by the persistence of the cold sensation in

manner of an after-image, and in part by the lessened sensibility of the nerves of best; a similar

explanation mutatis mutandis holds also for heat.

18. Extreme temperatures fatigue the sensory apparatus of both heat and cold. a. Hold a finger

in water of 45°c., the corresponding finger of the other hand in water which feels neither cold

nor hot (about 32°). After 10 seconds dip them alternately into water at 10°. The finger from the

water at 32° will feel the cold more strongly. b. Hold a finger in water at 10°, the corresponding

finger of the other hand in water a 32°. After 10 seconds dip them alternately in water at 45°.

The finger from the water at 32° will feel the heat more strongly.

19. Hold the hand for one minute in water of 12°c., then transfer to water of 18°. The latter will

at first feel warm, but after a time cold again. The water at 18° first causes a decrease in the

loss of heat or a slight gain but later a continued loss.

20. Fineness of temperature discrimination. a. Find what is least perceptible difference in

temperature between two vessels of water [p. 146] at about 30°c., at about 0°, and about 55°.

The finest discrimination will probably be found with the first temperature, if the discrimination

does not prove too fine at all these points to be measured with the thermometers at hand. Use

the same hand for these tests, always dipping it to the same depth. It is better to dip the hand

repeatedly than to keep it in the water. b. The different surfaces of the body vary much in their

sensitiveness to temperature. The mucous surfaces are quite obtuse. When drinking a

comfortably hot cup of coffee, dip the upper lip into it so that the coffee touches the skin above

the red part of the lip, or dip the finger into it; it will seem burning hot. Plunge the hand into

water at 5-10°c. The sensation of cold will be strongest at first on the back of the hand where

the skin is thin, but a little later will come out more strongly in the palm, where it will continue to

be stronger as it approaches pain.

On these general temperature experiments cf. the works of Weber and Goldscheider already

cited, also Hering. In Hermann's Handbuch der Physiologie, Vol. III, pt. 2, pp. 415-439.

Fechner, Element der Psychophysik, Vol. II, pp. 201-211.

SENSATIONS OF PRESSURE.

Apparatus. Bits of cork. Weights for minimal pressure. (These can be cut from rectangular

prisms of cork or elder-pith of equal area, and provided with bristle or hair handles and verified

upon a sensitive balance. The prism should be from 3 to 5 mm. square. The handle is made by

setting the ends of a piece of bristle or hair into opposite sides of the bits of cork or elder-pith,

thus giving the whole something the shape of a seal ring of which the cork is the seal and the

bristle the band. A series ranging from 0.002 to 0.02 grams would be convenient; but for the

experiment to follow is not necessary.) Two objects of equal weight, but unequal size; a large

cork and a small one, made of equal weight by loading the smaller with shot, answer very well.

Two metal disks of equal size and weight, e.g. dollar pieces; and two wooden cylinders three

quarters of an inch in diameter and one inch long. Vessels of water at normal temperature.

Weights for discriminative sensibility. (The last can readily be made by loading paper gun-shells

with shot. The following would be a convenient series: One hundred grams (two of this weight),

102.5, 103.5, 104, 105, 106, 107. The Cambridge Scientific Instrument Co., St. Tibb's Row,

Cambridge, England, manufactures a set, which can also be used for "muscle-sense" tests,

containing 30 weights and giving ratios ranging from about one-fourth to one fiftieth, at a price

of £5.)

On apparatus for sensations of pressure cf. Beaunis. Éléments de physiologie humaine. II, 579.

Eulenberg, Berlin. klin. Wochensch. 1869, No. 44 (See illustration and description of

Eulenberg's instrument in the Reference Hand-book of the Medical Sciences, Vol. I. p. 85.)

Dorhn. Zeitschr. f. rat. med., 3 R., X, 337. Bastelberger, Experimentelle Prüfung der zur

Drucksinn-Messung angewandten Methoden, Stuttgart, 1879. Jastrow gives in the American

Journal of Psychology, II. 54, a very inadequate description of a very satisfactory instrument.

See also notes on apparatus for the study of the Psychophysic Law to be given later.

21. Pressure points. Make an obtuse but extremely fine cork point (pyramidal in shape, for

example, the pyramid a quarter of an inch square on the base and of equal height), set it upon

the point of a pen or other convenient holder, or use a match whittled down to a fine point, or

even a needle. Choose an area on the forearm and test for its pressure spots somewhat as for

the hot and cold spots, but this time set the cork point as lightly as possible on point after point

of the skin instead of drawing it along. Two kinds of sensation will be felt; at some points a clear

feeling of contact with a sharp point will be felt, at others no feeling at all or a dull and vacuous

one. The first are the pressure points. Goldscheider describes their sensations on light contact

as "delicate," "lively," "somewhat tickling *** as from moving; a hair;" on stronger pressure, "as if

there war a resistance at that point [p. 147] in the skin, which worked against the pressure

stimulus;" "as if a small hard kernel lay there and was pressed down into the skin."

The first are more sensitive to small changes of pressure, and, though with sufficient increase

both give pain, their sensations retain their characteristics. They are closer together than the

temperature spots, and harder to locate; and the fact that our most frequent sensations of

pressure are from surfaces and not from points makes it difficult at first to recognize a pressure

quality in their sensations.

Cf Goldscheider, Neue Thatsachen über die Hautsinnesnerven. DuBois-Reymond's Archiv.

1885, Supplement Band, pp. 77-84.

23. Minimal pressure (or simple contact). Make weights that are just perceivable on the volar

side of the fore-arm and on the tips of the fingers. Try also if convenient the temples, forehead

and eye-lids. In applying the weights see that they are brought down slowly upon the surface of

the skin, that they touch equally at all points, and that their presence is not betrayed by motion

of the weight after it touches the skin. This can be done by using a pen-holder or small rod, with

its tip put through the ring of the weight, for laying it on. Compare the relative sensibility found

by this method with that found with Weber's compasses for the same parts, and note that the

latter requires discrimination, not mere perception. The stimulus needed to produce this faintest

sensation is known as the stimulus of the "Initial Threshold." See also experiment 28.

Cf. Aubert and Kammler, Moleschott's Untersuchungen, V, 145.

23. Relation of apparent weight to area of surface stimulated. Test with the equal weights of

unequal size. The smaller will seem decidedly heavier.

24. Discriminative sensibility for pressures. Have the subject lay his hand, palm upward, on

such a support as will bring his arm into a comfortable position and make his palm level, for

example a folded towel on a low table or the seat of a chair. (The matter of an easy position for

the subject is of cardinal importance in all psychological experiment, and is mentioned here

once for all.) Lay in his palm a piece of blotting paper just large enough to prevent the weight

from touching the skin. On this set a standard weight, e.g. 100 grams, and after a couple of

seconds replace it with an equal weight, or one heavier (or one lighter) e.g. 106 grams, allowing

that to remain an equal time. Require the subject to say whether the second weight is the same

or heavier, (or lighter, if a lighter is being used). Find the weight that can be distinguished from

the standard in 75 per cent. of the trials. The ratio between the difference of these and the

standard is the index of the sensibility. The ratio will probably be about 5:100. It is best not to

use both a lighter and a heavier in the same series; and with this method of testing the subject

should always guess, if he cannot discriminate. Be careful in putting on the weights that the

subject does not recognize a difference in the force with which they strike, also that suggestions

by difference of temperature or by sounds made in selecting the weights are avoided. This

method of determining sensibility is known as the "Method of Right and Wrong Cases." Cf. later

experiments on the Psychophysic Law.

25. Cold or hot bodies feel heavier than bodies of equal weight at a normal temperature. a. For

cold, take two dollar pieces warm one until it ceases to seem cold; cool the other to 10°c. Apply

alternately to the palm of the hand. The cold one will seem much heavier, perhaps as heavy as

two at the normal temperature, b. For heat, take two wooden cylinders of equal weight, heat

one to a high temperature by standing It on end in a metal vessel hosting in a water bath. Apply

the cylinders on end alternately to the back of the hand between the metacarpal bones or the

thumb and first finger. The hot one will seem heavier. [p. 148]

26. Pressure evenly distributed over a considerable area is less strongly felt than pressure

upon an area bordered by one that is not pressed. Dip the hand up to the wrist into water (or

better still into mercury) of normal temperature and notice that the sensation of pressure is

strongest in a ring about the wrist at the surface or the water, possibly stronger on the volar

than on the dorsal side. The ring effect is unmistakable when the hand is moved up and down

in the water.

27. Something of the refinement of the pressure sense in perceiving the unevenness of

surfaces may be seen by laying a hair on a plate of glass or other hard, smooth surface and

over it 10 or 16 sheets of writing paper. The position of the hair can easily be felt by passing the

finger tips back and forth over the surface.

28. Something might be said in support of the hairs as independent sense organs. The finest

respond with a distinct sensation of anticipatory touch, as it were, when they are moved, and

probably this accounts for a part at least or the differences between the fore-arm and finger tips

found in Ex. 21. Touch a few single hairs and observe the sensation.

Cf. Blaschko. Zur Lehre von den Druckempfindungen. Verhandl, d. Berllner physiol. Gessell.

Sitz., 27 März 1885, DuBois-Reymond's Archiv, 1885, p. 349.

On the general topic cf. Weber. op. cit.: Funke. Tastsinn und Gemeingefühl, Hermann's

Handbuch der Physiol., Vol. III. pt. 2, pp. 289-414.

II. -- STATIC AND KINÆSTHESIC SENSES.

This group of senses furnishes us with data respecting the positions and motions of our

members, and of our bodies as wholes. It includes senses whose existence or efficiency is

disputed e.g. (Innervation Sense and Muscle Sense) and others whose independence has

lately been asserted (e.g. Joint Sense and Tendon Sense.) This embarrasses somewhat the

selection of experiments, but those chosen are the ones that seem at present characteristic.

Many of the weightiest psychological inferences depend upon the sensations of motion and

position of the eyes. It seems best, however, to postpone the experiments upon these

sensations to the section upon vision.

RECOGNITION OF THE POSITION OF THE BODY AS A WHOLE.

Apparatus. A light wooden rod a yard long; a

tilting board and straps. For the last a board

seven feet long and 18 inches wide balanced

across a saw-horse will answer. At one end a

foot board should be fastened securely

enough to bear the weight of a man when the

board is in a vertical position. At the other end

a plumb line and semicircular scale should be

added so that the inclination of the board can

be read off at any instant. For holding the

subject securely upon the board when its

inclination is considerable, and the subject is

head downward it will be necessary to have a

couple of shoulder straps passing over the

subject's shoulders and fastening to stout

screw-eyes screwed into the board Itself or

into the foot board, and perhaps a breast

strap going about both the subject and the

board.

Cf. Aubert, Physiolgische Studien über die Orientierung. (translation with comments of Delage's

Études expérimentale sur les illusions statique et dynamiques de direction, etc.,) Tübingen,

1888, p. 41. [p. 149]

29. In this experiment it is especially desirable that the subject should know as little as possible

of the purpose of the experiment. Cause the subject to stand erect with his back against a wail.

Choose a point on the opposite mall about the height of his shoulders. Let him, look at it, and

then require him, having closed his eyes, to point to it as exactly as possibly with a light rod

held symmetrically in both hands. Cause him also to hold the rod vertically and horizontally in

the median plane; also horizontally parallel to the frontal plane. All these he will probably be

able to do with much accuracy, or if, as sometimes happens, he shows a "personal equation,"

his error will be constant.

30. a. Cause the subject to repeat the experiment, this time turning his head well to the left

after closing his eyes. Repeat, causing the subject to turn to the right. In both cases an error of

about 15° will be observed, the subject pointing too far by that amount in the direction opposite

to that of the turning of the head. The subject will be able to hold the rod vertical or horizontal

without error. b. Cause the subject to hold the rod in what he thinks is a horizontal position in

the median plane, when his head is thrown well back; when bowed well forward. Illusions like

those observed above will result. c. Cause the subject to hold the rod in what he thinks is a

horizontal position, parallel to the frontal plane, when his head is leaned to the right; when

leaned to the left. Illusions similar to those in the previous experiments will appear. d. Repeat

experiment a, but instead of having the subject point to the designated object, have him walk

toward it keeping his shoulders square, his eyes shut, and his head turned to one side. He will

walk more and more too far toward the side away from which his head is turned. In all these

cases judgment of one cardinal direction in space alone is affected, the other two show little or

no errors.

31. After some practice and with attention to the sensations, the illusion of Ex. 30 takes another

form, namely, that the body has turned a few degrees in the same direction as the head. The

subject can now point to the chosen object; but, if required to set the end of the rod against his

breast so that it shall be horizontal and perpendicular to the line joining his shoulders, he will

make an error of about 15° in the direction of the motion of the head. A similar illusion may be

found for the other directions of head turning, ii tried under proper conditions e.g. when hanging

by the hands with the arms somewhat bent.

32. The illusion is due, at least in cases a and b Ex. 30, to sensations of the position of the

eyes. As may easily be observed upon tiny other person, the eyes turn further than the head in

the direction in which it is turned. From the eyes we judge the position of the head, and thus

overjudging it, point too far in a contrary direction in trying to point to the required object. The

illusions can be produced by motion of the eyes alone. a. Holding the head erect and taking

pains not to move it when moving the eyes, turn the closed eyes as far as possible to the right

or left and then try to point to some determined object. An error like that in Ex. 30 will be

observed. Turning of the eyes upward or downward has a less satisfactory result. Instead of

closing the eyes they may be kept open if an opaque screen is held close before the face. b.

Repeat a and b of Ex. 30, voluntarily turning the eyes as far as possible in the direction

opposite to that or the turning of the head. The original error will disappear or be found to have

changed its sign.

33. Another set of illusions regarding the position of the body as a whole in space depend in

large measure on the distribution of pressure on the surfaces of the body the direction of

pressure of the movable viscera and the blood. Secure the subject properly upon the tilting

board, and have him close his eyes. Start with the board vertical, (head up). [p. 150]

The subject will probably announce that he is then leaning forward slightly. Turn him slowly

backward and require him to announce when he is vertical (head up), when he is tilted

backward at an angle of 45° from the vertical, when at an angle of 60°, when at 90°, when at

180°. Two classes of illusions will be found; angles of less than 40° will seem too small; those

from 40° to 60° will be rightly judged; those beyond 60° will seem too large. The subject will

probably say that he is vertical, head downward, when he is yet 30-60° from it.

SENSATION OF ROTATION.

Apparatus. -- Rotation Table. This can be made well enough for the experiments given by

laying a 7-foot board across an ordinary turning chair or screw stool without a back. The last

must turn without appreciable noise or jar. Many of these experiments could be made perfectly

well by twisting the ropes of an ordinary swing.

34. Lay the board across the stool and let the subject be seated upon it with closed eyes and

blindfolded if necessary. Turn the stool slowly and evenly in one direction or the other. The

subject will immediately recognize the direction and approximately the amount of rotation when

the rate is as slow as 2° per second, or even slower. After continued rotation at a regular rate

the sensation becomes much less exact or entirely fails. This fact has been generalized by

Mach in the law that only change of rate, not continuous rotation is perceived. With an

exception in the case of uniform rates for short times, this is accepted by Delage. After some

pauses and short movements in one direction and the other, the subject may become quite lost

and give a totally wrong judgment of the direction of motion, if it is slow.

35. Let the subject be seated as before. a. Rotate him a little more rapidly for half a turn, and

then stop him suddenly. A distinct sensation of rotation in the opposite direction will result. b.

Repeat and when the illusory rotation begins, open the eyes. IT immediately ceases. Close the

eyes again and it returns.

36. a. Repeat experiment 35a, letting the subject give the word for stopping. At the same

instant let him incline his head suddenly backward or forward or lay it upon one shoulder or the

other. The axis of rotation of the body will appear to change in a direction opposite to that of the

inclination of the head, i.e., if the head is inclined to the right, the axis, seems to incline to the

left. The feeling is as if the body were rotating; in the surface of a cone in a direction contrary to

that of the first rotation. The head dictates the apparent axis of rotation. The same illusion

occurs if the head is inclined during the actual rotation and straightened at the word for

stopping. Turning the head to right or left introduces no such illusions, because it does not

change the axis of rotation of the head. The illusion comes out with very disagreeable strength

when the rotation is rapid and the subject changes the position of his head during the rotation,

b. Let the subject lie upon his side and rotate him rather rapidly till the sensation of rotation

becomes faint or disappears. Then let him turn suddenly upon his back or upon his other side.

The first brings the rotation about a new axis, and it is felt in its true sense, while the rotation

about the previous axis is felt in its reverse sense; the second reverses the direction of motion

completely and produces a correspondingly powerful sensation.

The change of the apparent axis of rotation with the change of position of the head points to the

location In the head of the organ by which such sensations are received. For the experiments

by which the semicircular canals are indicated as this organ see the literature cited below.

37. Another illusion of rotation (Purkinje's dizziness) is due to the [p. 151] motion of the eyes.

Let the subject whirl rapidly on his heels with his eyes open till he begins to be dizzy; while he

whirls the objects about him will seem to be turning in the opposite direction. Let him then stop

and look at an even surfaced wall while the experimenter carefully observes his eyes, picking

out a fine blood-vessel, or some other clearly marked fleck or spot as a point at which to look.

To the subject the surrounding objects will seem to continue to move in the same direction as

before, i.e., in a direction contrary to his previous rotation; the experimenter will see the

subject's eyes executing slow motions in one direction (in the direction of the original motion of

the subject) alternating with rapid motions in the other. The subject himself may be able to

perceive a corresponding irregularity of motion in the spots upon the wall at which he looks.

The illusion rests upon the subject's unconsciousness of the slow motions of his eyes. It is not

improbable that these eye motions and the sensations of attempted restoration of equalibrium

[sic] in other parts of the body are reflexly caused by the disturbance in the semicircular canals.

It should be noticed that this illusion is the exact reverse of that found with closed eyes in Ex.

36. There the subject feels a rotation of his own body contrary to that it previously received. If

he was turned at first in the direction of the hands of a watch, on being stopped he would seem

to be turning in a direction contrary to the hands. If these motions were transferred to objects

about him, they would, during the rotation, seem to move contrary to the hands and after

stopping in the direction of the hands. In the Purkinje experiment the motion of objects is not

thus reversed.

SENSATION OF PROGRESSIVE MOTION.

39. So far as progressive motions do not partake of rotation the sensations which they give us

are probably combinations of sensations from several different sources or sensory judgments

based thereon. For them, as for the motions of rotation, the principle holds that we perceive

changes of rate of motion, and not uniform motion; as long as the motion remains uniform we

can by an effort of imagination conceive ourselves to be moving in either direction or to be

standing still, except for the jarring. The apparatus for the study of these phenomena will be

found in railroad trains and elevators.

On the sensations of this and the preceding sections, cf. Aubert, translation of Delage above

cited; Mach, Bewegungs-Empfindungen, Leipzig, 1875; Brown, On Sensations of Motion,

Nature, vol. XL., 1889, p. 448, ff.

MUSCLE SENSE, Kraftsinn.

The real muscular sensations are probably those of pain, fatigue and the like, and are of

relatively minor importance for psychology, but the term "muscle sense " has been used to

designate that sense by which lifted weights are perceived, and is here used in that sense.

Apparatus. Set of test weights somewhat like those used for the pressure sense, but less

different one from another, (For example 100 grams (two of this weight), 101.6, 102, 102.2,

102.5, 102.8, 103,3[sic]). Weight of 2 or 3 kg.

39. Discriminative sensibility for lifted weights. a. Let the subject stand at a table of convenient

height. Place within easy reach of his right hand, and near together, one of the standard

weights (e.g. 100 gm.) and a weight to be compared with it, either the other standard or a

heavier (or lighter) one. Let the subject lift one after the other, taking care to lift them in the

same way, at the the [sic] same rate and to the same height, and give a decision as to which is

the heavier (or the lighter). Find the weight that can be distinguished from the standard in 75

per cent. of the trials. As before the ratio between the difference of these and the standard is

the index of the discriminative sensibility. The ratio will probably be about 2.5:100. b. Repeat

the experiment letting the subject lift with one hand the standard and with the other [p. 152] the

weight to be compared, keeping the same hand for each during each series of trials. Note the

discriminative sensibility as before; the discriminations will be much less fine.

In these experiments the sense of pressure might be expected to co-operate, but when it is

excluded or put at a relative disadvantage, the sensibility for differences of lifted weights is not

diminished. Weber's method of excluding the pressure sense was to wrap the weights in pieces

or cloth and lift them by the four corners together. The pressure on these corners can be

changed at will irrespective of the heaviness of the weight lifted. Compare the discriminative

sensibility found for pressure with that found for lifted weights.

40. Careful experiments on the method of such discriminations shows that the determining

factor is the rapidity with which the weight rises as it is lifted. The following experiment is one of

those upon which this conclusion rests. After having performed the second part of Ex. 39

compare the standard weight with a very much heavier weight, e.g., 2 kg., with all the

circumstances of actual careful judgment. Practice this judgment thirty times, leaving a larger

interval of time between the individual comparisons than between liftings of the weights

compared. Then at once return to the smaller weights, giving the standard to the same hand as

before and the weight to be compared to the hand that has just been lifting the 2 kg. Not only

will the weight before just recognizably heavier seem considerably lighter than the standard, but

also still heavier weights will seem so. This time the tests must be few, not more than three or

four. If more should be desirable, practice the comparison, of the standard and 2 kg. weight

again ten times before taking them. By the practice the nervous centres discharging into the the

[sic] muscles that raise the 2 kg. weight become accustomed to a larger discharge than that

required for the small weights and do not at once re-adapt themselves, but supply too great a

discharge, the weight rises with greater rapidity than the standard and is consequently

pronounced lighter.

Cf. on Muscle Sense. Weber, op. cit.; Müller und Schumann. Ueber die psycholodgischen

Grundlagen der Vergleichung gehobener Gewichte, Pflüger's Archiv, Bd. XLV, 1889, pp. 37-

112; James, Psychology. II, pp. 189 ff.

Cf. also the experiments and references on the Psychophysic Law.

INNERVATION SENSE.

Apparatus. Blackboard and chalk.

41. The evidence most frequently offered in support of this sense is clinical and therefore

beyond the scope of this course. Experiments like the following have been brought forward, but

their interpretation has been disputed. a. Stand erect before the black board with the eyes

closed and coat off, if it interferes with free motion of the arms. Draw with each hand a

conventional leaf-pattern like those in the annexed cut drawing from a to b in both cases. In

drawing try to make the lobes of the leaf of equal size, like those in Fig. A; draw each with a

single simultaneous "free hand" motion of the arm, that is, draw each with a single volitional

impulse directed equally to the two sides -- the last point is important. First draw a pair of

leaves beginning them with the hands before

the shoulders at the same height; the result

will be approximately like Fig. A. Next draw a

pair with one hand a foot lower than before;

the result will be like Fig. B. b. Bring the

hands again to the position used in drawing

Fig. A, and draw a pair or leaves having their

spices right and left. The leaves will be

symmetrical. Next begin with one hand about

a foot farther away from the median plane

than before and the other at it, but both at the

same level. Draw as before; asymmetrical

leaves will be the result. Repeat the drawing

a number of times, sometimes raising or

extending one arm, sometimes the other. In

general it will be found that notwithstanding

the intention to make equal movements of the

hands, the motions of further extension in the

extended arm and of further flexion in the

flexed arm are too short and those in the

contrary direction in each case too long. The

argument founded on this experiment runs as

follows: We think that our hands execute

equal movements, when they do not, because we are conscious of willing equal movements,

and unconscious or only inexactly conscious of those actually made. If on the contrary we

perceive motion of members by the skin, joint and muscle sensations that accompany their

motion (as the opponents of the Innervation Sense believe) we ought to know the extent to

which our hands are moved each time and not fall into the illusion that we find in these

experiments.

42. Lay the hand palm downward on the edge of the table or on a thick book so that the last

three fingers shall be supported and held extended while the thumb and fiirst finger remain free.

Bend the first finger considerably at both the inner joints, and hold it in position with the other

hand. The finger tip is still movable as will be found on touching it, but it is anatomically

impossible to move it voluntarily. When, however, the effort is made to move it (the eyes being

closed) there is a sensation of motion, though no actual motion is possible.

43. Of experimental evidence against the Innervation Sense there is more. Müller's experiment

(No. 40) seems conclusive against it; for if there were any sensation of nervous discharge, we

ought to know that when we go from a very heavy to a light weight the discharge is

disproportionate; but we do not. That the feeling of effort is or: peripheral and not central origin

is shown by such experiments as this of Ferrier's. a. Hold the finger as if to pull the trigger of a

pistol. Think vigorously of bending the finger but do not bend it; an unmistakable feeling of effort

results. Repeat the experiment and notice that the breath is involuntarily held, and that there

are tensions in other muscles than those that would move the finger. Repeat the experiment

again, taking care to keep the breathing regular and other muscles passive. No feeling of effort

will now accompany the imaginary bending of the finger. b. Lay the fore-arm entirely relaxed in

the scale pan of an ordinary balance (or better still of a spring balance) and put in weights

enough to compensate it exactly. Remain with closed eyes keeping the arm relaxed. It will after

a little overbalance the compensating weights, showing that at first it was not wholly relaxed. An

Inervation[sic] Sense, if we had one, ought to prevent such an illusion.

Cf. on the Sense of Innervation, Wundt. Physiologische Psychologie; I. 397. ff.; Sternberg, Zur

Lehre von den Vorstellungen über die Lage unserer Glieder, Pflüger's Archiv, XXXVII 1885, 1.

Loeb. Untersuchungen über die Orientirung[sic] im Fühlraum der Hand und im Blickraum,

Pflüger's Archiv. XLVI. 1-46, (but see also criticisms of James, Psychology, II. 516, and of

Christine Ladd-Franklin, Amer. Jour. Psy., II, 653); James. Psychology, II, pp. 486, ff.; Ferrier,

Functions of the Brain, pp. 382 ff.,(English Ed.); Funks, op. cit. [p. 154]

SENSATIONS OF MOTION, (Joint Sense).

Apparatus. Hinged board for passive flexion

of the elbow. The accompanying cut will give

some idea of the construction of such a

board. The thin board on which the fore-arm

rests (50 cm. long by 8-10 wide) is hinged at

one end to the base board. At the other end a

cord is fastened that runs over a pulley upon

the top of a stout post. On the end of the cord

a weight is hung to counterbalance the weight

of the fore-arm. A scale (e.g. a piece or mm.

paper) on the post near the weight enables

the experimenter to read off the distance

which the end of the arm-board is raised or

lowered. It is essential that the hinge and

pully[sic] work easily and without jar. The

above is simply one way of accomplishing the

result; others will occur to those for whom this

construction is inconvenient.

44. Passive flexion at the elbow. Let the subject rest the fore-arm flat upon the arm board,

bringing the elbow over the hinge, and close his eyes; raise the fore end of the arm-board

slowly by pressing down upon the counter weight, and require the subject to announce when

he first perceives the motion of his fore-arm. It is extremely important not to mistake the

sensation of increased pressure or of jar for that of motion. With the dimensions given above,

one degree of angle corresponds to about 8.7 mm. The same apparatus may be used for

extension as well as flexion.

45. Active flexion of the last joint of the finger. The joint sensations or the fingers are less fine

than those of the elbow, but are more convenient for demonstration of active flexion. Fasten a

piece of straw, with court-plaster or otherwise, to the finger nail of the middle finger, and cut it

off at such a length that the distance from the joint of the finger to the end of the straw shall be

115 mm. With that radius 2 mm. corresponds to about 1° of angular measure. Rest the hand on

a thick book letting the last joint of the finger extend beyond the edge. Set up a millimeter scale

at right angles with the straw. Close the eyes and make the least possible region of the finger at

the last joint, having an assistant note its extent on the scale. Between one and two degrees

will probably be the least possible voluntary movement. Close attention will, probably in both

these cases, locate the chief sensation in the joint. For the more rigorous experiments required

to show its character clearly and to prove its location see the following:

Goldscheider Untersuchungen über den Muskelsinn. Do Bois-Reymond's Archiv, 1889. pp. 369

ff. and 540, also Supplement-Band, 1889. 141 ff.

SENSATIONS OF RESISTANCE.

Apparatus. Two or three kilogram weight and string. Vessel of mercury.

46. a. Hold the weight by the strong so that it hangs a few inches above the floor, with the arm

extended; Lower the weight rather rapidly till it rests on the floor. As it strikes, an illusion of

resistance to further motion will be perceived. This is due to the unexpected strain put upon the

muscles that lower the arm by the tension of those that have been holding the weight. The

feeling of resistance is probably a [p. 155] joint-sensation, b. Something similar is observed on

pouring a quantity of mercury from one vessel to another.

Cf. Goldscheider, op. cit.

BILATERAL ASYMMETRIES OF POSITION AND MOTION.

Apparatus. Two medium sized corks. A millimeter scale at least one meter long. This can easily

be made by pasting millimeter paper upon a smooth wooden slat. A convenient scale has a

right angled triangular section. In use this stands upon the short side of the triangle, the long

side is next the subject, the hypothenuse[sic] next the observer. The millimeter paper is pasted

along the upper edge of the side next the observer.

47. Apparently symmetrical positions of the two arms. Hold a cork between the thumb and first

two fingers of each hand. Close the eyes and bring the two corks together at arms length in the

median plane before the face, having an assistant note the approximate amount and direction

of the error. The corks should he brought together rather gently so as not to betray the

character of the error to the operator, but the motions of the arms by which they are brought up

nearly to contact should be free and sweeping. The error will probably be found rather constant

in direction until the operator learns to correct it. Try bringing the corks together above the

head, and also in asymmetrical positions.

48. Let the subject seat himself at a table with the millimeter scale before him. Set a pin in the

middle of the scale and bring the pin into the median plane of the subject and make the scale

parallel to his frontal plane. Let the subject place his forefingers on either aide of the pin, and

with closed eyes, try to measure off equal distances by moving each outward along the scale.

Note the result in millimeters; for this it may be convenient to mark the middle point of the

finger-nails with an ink-line. A constant excess in the motion of one hand or the other will be

found. It is important that the subject should not open his eyes till his fingers are removed from

the scale; for he will find it difficult not to correct his error if he knows its nature. The finger tips

should rest lightly on the scale and the motions should be made by a single impulse; if they are

too slow and the subject attends to his sensations of position, the errors will be small and

uncertain. The greatest errors will probably be round for distances of 20 to 50 cm. from the

median plane. The left hand generally makes the greater excursion in right handed persons not

mechanics. b. Repeat the tests having the motions of the hands made successively instead of

simultaneously. The constant difference between the hands will not appear. c. Operate

somewhat as in a, but this time let the experimenter move one of the subject's hands passively

while the subject himself tries to move the other at the same rate and to stop instantly when the

passive motion stops. Try passive motions of the right as well as the left hand. The errors found

will generally resemble those of a. d. Let the subject start with his right or left hand 20 cm.

toward its own side of the median plane, and try to measure off equal distances on either side

of that point, using tile same hand for both distances. Indicate the point of departure with a pill

as before and mark off with another the standard distance to be reproduced. Distances outward

will be made too large, distances inward too small. In all these experiments with closed eyes

we seem inclined to judge distance rather from the intention of equal motion and the

continuance of motor sensations for equal times than from the actual peripheral sensations.

Cf. Hall and Hartwell. Bilateral Asymmetry of Functfon. Mind. Vol. IX; Loeb, Pflüger's Archiv,

XLI, 1887, pp. 107-127, also Pflüger's Archiv, XLVI, 1890, pp. 1-46.

(To be Continued.)

III.--TASTE AND SMELL.

SENSATIONS OF TASTE.

Apparatus. A potato and an apple; standard solutions of sweet, bitter, sour and salt; camel's-

hair brushes; battery and zinc electrodes. The standard solutions should be made of two

strengths, the stronger for testing the individual papillæ and the weaker for finding the least

proportion tastable. The following proportions of tastable substances and water are convenient.

Stronger solutions: Sugar. 40:100; Quinine, 2:100; Tartaric Acid, 5:100; Salt, saturated solution.

Weaker solutions, (for which the water itself should be without taste): Sugar, 5:100; Quinine,

2:100 000; Tartaric Acid, 5:1000; Salt, 2:100. Special solutions of Sugar for Ex. 52: 20:100,

18:100, 16:100, 14:100, 12:100, 10:100.

49. Much of what is commonly called taste is really taste plus smell or touch or both. With' the

eyes shut and the nostrils held try to distinguish, by taste alone, between small quantities of

scraped apple and potato, placed upon the tongue.

50. Distribution of the Organs of Taste. a. Using the weaker solutions and operating with a

mirror or on another person, find out as nearly as you can in what part of the tongue the

strongest sensations are produced by each. Test the tip, the sides, the back and the middle,

putting on the solutions with a camel's-hair brush and rinsing the mouth as often as necessary.

Try also the hard and soft palates. b. Dry the tongue with a handkerchief and test the individual

fungiform papillæ with the stronger solutions, applying them with fine camel's-hair pencils. It will

be found possible to get taste sensations from the single papillæ, though perhaps not all four

from each. Rinse the mouth as needed, c. Test the surface of the tongue between the papillæ

and observe that no taste sensations follow.

On a cf. Rittmeyer: Geschmacksprüfugnen. Göttingen Diss. 1885. On b and c cf. Oehrwall,

Untersuchungen über den Geschmackssinn, Scandinav. Archiv f. Physiol. Bd. II. 1890, pp. 1-

69; see also abstract by the author in the Zeitschrift f. Psych. Bd. I. 1890. p. 141.

51. Minimal taste. a. Find what is the greatest dilution of the weaker solutions in which the

characteristic taste can still be recognized. The same quantity, e.g., half a teaspoonful, should

be taken into the mouth at each trial and may be swallowed with advantage. Rinse [p. 304] the

mouth as required. The following are the proportions given by Bailey and Nichols for male

observers: Quinine, 1:390 000; Sugar l:199; Salt, 1:2240; Sulphuric Acid, which they used

instead of Tartaric, the proportion was 1:2080. B. The intensity of the sensation end the

greatest dilution still tastable depend on the number of taste organs stimulated. Take a portion

of one of the solutions of just testable strength found in a, add an equal quantity or water and

take a large mouthful of the mixture. The characteristic taste will still be perceived, perhaps

more strongly than before.

On a cf. Bailey and Nichols, The Delicacy of the Sense of Taste. Nature. XXXVII, 1887-88, 557;

and Lombroso und Ottolenghi, Die Sinne der Verbrecher. Zeitzchrift für Psychologie Bd. II.

1891. Pp. 346-48. Camerer. Die Grenzen der Schmeckbarkeit von Chlornatrium in wässrigar

Lösung. Pflüger's Archiv. II, 1869. 322. On b cf. Camerer, Die Methode der richtegen und

falschen Fälle angewendet auf den Geschmackssinn. Zeitschrift für Biologie, XXI, 570..

52. Discriminative sensibility for taste. For a rough determination test with the solutions of sugar

indicated above, taking first a smell quantity or the standard 20% solution, then an equal

quantity (the equality is important) of one of the weaker solutions, or first one of the weaker and

then the standard, until a solution is found that is just recognisably different from the standard.

Make this determination several times. The excess of sugar in the standard solution over the

amount in the solution Just observably weaker set in a ratio to the total percentage of sugar in

the standard measures the sensibility. Some experimenters may be able to distinguish the 18%

from the 20% solution; their sensibility would then be expressed by the ratio 2:20.

On such experiments as this cf. Keppler Das Unterscheidungsvermögen des Geschmacksinnes

[sic] für Concentrationsdifferenzen der schmeckbaren Körper. Pflüger's Archiv, II, 1869, 449.

53. Electrical Stimulation. a. Using a constant current and two zinc electrodes one above, the

other under the tongue notice the sour taste at the positive pole and the alkaline at the

negative.

On the sensations of taste in general cf. von Vintschgau, Physiologie des Geschmackssinne,

Hermann's Handbuch der Physiologie, III. (pt. 2) pp. 145-224; Oehrwall, op. cit. On acid tastes

and chemical composition cf. Corin, Action des acides sur le goût, Archives de Biologie, VIII,

fasc. 1.

SENSATIONS OF SMELL.

Apparatus. Essence of cloves; olfactometer

or Zwaardemaker; camphor gum; yellow wax;

a dozen small wide mouthed bottles. The

essence of cloves is made by adding one part

of oil of cloves to fifteen parts of alcohol[1]

and may be diluted with water, itself odorless,

to make the solutions required in Ex. 54. For

that experiment dilutions of the essence that

will give the following proportions of oil of

cloves will be found convenient 1:50 000;

1:100 000; 1:200 000; 1:300 000; 1:400 000;

1:500000. The olfactometer of Zwaardemaker

in simple clinical form may be bought of

Mechaniker Darting Bank, Utrecht, at 1.50

mk.; but [p. 305] its construction is so simple

that it may easily be made in the laboratory. It

will be most convenient if made double as

shown in the accompanying cat. The

instrument consists of a light wooden screen, say six inches square, provided with a handle

below for easier holding. Through this screen, a little below the middle, a hole an inch and a

half in diameter is bored, and fitted with a large cork. The cork in turn is pierced with two holes

side by side an inch apart and or such size as to fit tightly upon the glass tubes next to be

mentioned i.e. about 7 mm. The glass tubes should be long enough to leave 10 cm. free behind

the screen and about 3 cm. free in front. The front ends are bent upward at right angles for

insertion in the nostrils. The odorous substances used in this instrument are applied in the form

of tubes that slide over the glass tubes behind the screen. The simplest and best for persons or

normal keenness of smell are pieces of red rubber tubing 10 cm. long and of such bore as just

to slide freely over the glass tubes (8 mm.). These pieces of rubber tubing should themselves

be slipped into pieces of tight fitting glass tubing so as to prevent the spread of the odor from

their outer surface. For Ex. 57 another odor tube, this time of yellow ware will be needed. This

can easily be made by placing a glass tube (of the size of the air tubes used in the

olfactometer) inside a tube such as is need to cover the rubber odor-tubes and filling the space

between them with melted wax. The inner tube may then be warmed by running hot water

through it till it can be withdrawn. The principle upon which the instrument works is this, namely,

that the intensity or the odor varies directly as the surface of odorous substance exposed.

When the odor-tubes are slipped onto the glass tubes of the olfactometer and pushed back

until their ends are flush with those of the glass tubes, the air inhaled through the latter contains

few or no odorous particles because no odorous surface is exposed. When, however, the odor-

tubes are pulled a little away from the screen so that they extend over the ends of the glass

tubes, they expose the odorous surface inside them to the current of air inhaled. The strength

of the odor is proportional to the area exposed, or since the bore does not change, to the length

of odor tube that extends beyond the glass tube. This last can be conveniently measured by a

scale (e.g. centimeters and half centimeters) scratched on the inner glass tubes. The length of

odor tube corresponding to a just observable odor will, of course, differ with different tubes,

from person to person, and with the temperature, but tubes of red rubber are reported to give

satisfactory results both as to original intensity and the constancy with which they keep their

odor through considerable periods of time. The length of red rubber odor tube required by

Zwaardemaker himself for a just observable odor at 18°C. is 7 mm. In use the upward turned

end of one of the glass tubes is inserted in the forward part of the nostril and the subject draws

his breath in the way most natural to him in smelling -- the proportion of odorous particles is

greater, however, when the current of air is slow than when it is rapid. The inside of the glass

air tubes will need to be cleaned of adhering odorous particles from time to time. [p. 306]

On the olfactometer and its method of use see the following papers cf. Zwaardemaker: Die

Bestimmung der Geruchschärfe, Berliner Klin. Wochenschrift, XXV, 1888. No. 47, p. 950

(abstract of the same, British Med. Journal, 1888, ii, 1295); also Lancet, London, 1889, i, 1300.

On an improved form adapted to liquid substances (and thus to substances of known and

relatively simple chemical composition) see, Compensation von Gerüchen mittelst des

Doppelreichmessers, Fortschritte der Medicin, VII, 1889, 725 ff.

54. Minimal odors. The keenness of smell may be tested with dilute solutions or odorous

substances or with the olfactometer it will be instinctive to test by both methods. a. Tests with

solutions. Pour small quantities of the solutions of oil of cloves described above into little wide-

mouthed bottles, filling each to about the same height. Mark all in an inconspicuous manner.

Set the bottles on a table a foot apart in a place where there is moderate circulation of air, in

the order of the strength of their solutions, beginning with the water and following with the

weakest solution and so on. Require the subject to smell of the bottles in succession without

lifting them from the table, beginning with the water, and to indicate that in which he first

recognizes a characteristic odor. If the solutions stand for any length or time where they are

subject to evaporation it will be safer to prepare fresh ones before undertaking a new test.

Other precautions will suggest themselves, as for example, the use of bottles of the same size

and shape, and care in filling them that some of the solution is not left clinging near the mouth.

The just observable solution will probably be found to lie between the l:100 000 and l:400 000.

b. Tests with the olfactometer. Test the sides of the nose separately. Push the odor tube on till

its end is flush with that or the glass tube, insert the bent end of the latter into the nostril as

described above, and gradually lengthen the exposed surface of the odor-tube till its odor is just

discernable. Note in millimeters the length exposed.

On a cf. Bailey and Nichols, The Sense of Smell, Nature, XXXV 1886-87, 74; Lombroso and

Ottolenghi, op. cit. under Ex. 51. On b cf. Zwaardemaker, Sur la norme de l'acuité olfactive

(olfactie), Archives Néerlandaises, T. XXV, pp. 131-148.

55. Discriminative sensibility for odors. Using the double olfactometer with both odor-tubes

drawn out far enough to give an unmistakable odor, but not too strong a one, say both drawn

out 6 cm., find how far one or the other must be drawn out (or pushed back) to make the odor

which it gives just observably stronger (or weaker) than that of the other, The test should be

made with one side of the nose only, (there is frequently a difference in sensitiveness between

the two sides, due to mechanical obstruction or other cause) unless for some reason a bilateral

form of experiment is desirable. Try a number or times, but be careful to avoid fatigue.

56. Fatigue of smell. a. Hold a piece of camphor gum to the nose, and smell of it continuously,

breathing in through the nose and out through the mouth, for five or ten minutes. A very marked

decrease in the intensity of the sensation will be observed, reaching perhaps even to complete

loss of the odor. b. It is important, however, to observe that fatigue for one substance does not

cause obtuseness for all other substances, though it does for some. Smell of some essence of

cloves and of some yellow wax, then fatigue for camphor as in a and smell of the essence of

cloves and of the wax again. The odor of the wax will probably be fainter, that of the essence of

cloves unaffected.

Cf. Aronsohn, Experimentelle Untersuchungen zur Physiologie des Geruchs, DuBois-

Reymond's Archiv, 1886, pp. 3216-57.

57. Compensation of odors. a. Experiment with the olfactometer on one side of the nose as

follows. Hold against the end of the rubber odor-tube another odor-tube of wax, partly covered

on the inside by a glass tabs of the same size as that used in the olfactometer in such a way

that the air must pass through both to reach the nose. Then gradually increase the length or the

rubber tube exposed till the odor of the [p. 307] wax is no longer perceived. If the experiment is

carefully performed a point may be found where the two odors nearly compensate and the

resulting sensation approaches zero. If the rubber is lengthened beyond this point its odor

overpowers that of the wax; if it is shortened it is overpowered by that or the wax. A mixture of

the odors is hardly to be found. b. Repeat the experiment, using the double olfactometer with

rubber on one tube and war on the other. The compensation will be observed as before though

each side of the nose receives a separate stimulus. If the two sides of the nose are not equally

keen scented the proportions of the tubes that give compensating odors will not be the same as

before. Care should of course be taken to avoid fatigue.

Cf. Zwaardemaker, Compensation von Gerüchen mittelst des Doppelreichmessers. Fortschritte

der Medicin, VII, 1889, No. 18. pp. 721-731.

On smell in general, beside the literature already cited cf. von Vintschgau, Physiologie ded

Geruchssinnes. Hermann's Handbuch der Physiologie, III, (pt. 2) pp. 225-286, and the literature

there cited.

IV. -- HEARING.

SOUNDS IN GENERAL.

Apparatus. A watch, 2 yards of three eights[sic] inch rubber tubing a tuning-forks (the ordinary

A forks sold at music stores at 25 cents each will answer if a couple are chosen that prolong

their sound well), and a hammer. A watch is not an ideal instrument for testing acuteness of

hearing, but has the advantage of ready accessibility and simplicity in use.

For other special instruments for testing acuteness of hearing cf. Hensen, Physiologie des

Gehörs, Hermann's Handbuch der Physiol. III. pt. 2, pp. 119-120 and the references there

given; also Jacobson, Ueber Hörprüfung und über ein neues Verfahren zur exacten

Bestimmung der Hörschwelle mit Hülfe elektrischer Ströme, Du Bois-Reymond's Archiv, 1888,

189. For apparatus for testing the discriminative sensibility for sounds cf. (for noise) Starke, Die

Messung von Schallstärken Wundt's Philos. Studien, III, 1886, 266, and Zum Mass der

Schallstärke Ibid. V, 1889, 157; (for tone) Wien, Ueber die Messung der Tonstärke, Inaug. Diss.

Berlin, 1888, also in Wiedmann's Annalen, XXXVI, 1889, 834-857.

On hearing in general cf, Helmholtz, Sensations of Tone, Eng. tr. by Ellls, Hensen, op. cit.;

Stumpf, Tonpsychologie; Wundt, Physiologische Psychologie.

58. Minimal sounds. a. Experiment in a large room, furnished (to lessen the echoes) and as

free as possible from noise. Let the subject be seated with his side toward the experimenter, his

eyes closed and his ear upon the other side plugged with cotton. Let the experimenter then find

what is the greatest distance at which the subject can still hear the tick of a watch held at the

level of his ear and on the prolongation of the line joining the two. This is easily done with

sufficient accuracy by drawing a chalk line on the door, marking off feet or meters and fractions

upon it and estimating by eye the point of the line directly under the watch. Try several times for

each ear both when the watch is being brought toward the ear and when it is being carried

away. The experimenter should from time to time cover the watch with his hand to discover

whether or not the subject really hears or is under illusion. For normal ears the distance found

may vary from 2.5 m. to 4.6 m. and may even rise to as much as 9 m. b. The subject should

notice in this experiment the very marked intermittences of the sound when just upon the limit

of audibility. It will for a few seconds be heard above doubt and a few seconds later will as

certainly not be heard.

On a cf. von Bezold, Schuluntersuchungen über das kindliche Gehörorgen, Zeitsch,[sic] f,[sic]

Ohrenheilkunde, XIV, 1884-85, and XV. 1885-86. This paper gives the results of numerous

tests on Munich school children, not only with the watch but also with the acoumeter of Politzer

and with whispered speech. On b cf. Urbantschitsch, Ueber eine Eigenüimlichkeit der

Schallempfindungen geringster Intensität, Centralblatt f. d. medic. Wissensch., 1875, 625; N.

Lange, Beiträge zur Theorie der sinnlichen Aufmerksamkeit und der activen Apperception,

Wundt's Philos. Studien, IV 1888, 390; Münsterberg. Schwankungen der Aufmerksamkeit,

Beiträge zur experimentellen Psychologie, Heft 2, 1889, 69.

59. Auditory fatigue. Cause an assistant to strike once with a hammer on the floor, or to clap his

hands. With the ears open a single [p. 308] sound, or at most a single sound and transient

echoes are heard. If, however, the ears are kept closed with the fingers till half a second or

more after the stroke (the time may easily be fixed by rapid counting), the fainter echoes will be

heard like a new stroke. In the first case, fatigue from the original sound deadens the ears to

the fainter echoes, though they may still be heard by attentive listening; in the second case they

are more strongly heard because the closed ears are unfatigued. The sound produced by the

simple opening of the ears without any objective stroke will be less if the finger is not put into

the ears, but presses the tragus back upon the opening. b. Insert in the openings of the ears

the ends of a rubber tube. Strike a tuning-fork and set it upon the tube at such a point that It

sounds equally Intense to the two ears. (The sound will then probably appear to be located in

the head midway between the ears -- at least not nearer one than the other). After a few

seconds strike the tuning-fork again, pinch the tube on one side, say the left, so as to shut off

the sound from the ear on that side, set the tuning-fork on the tube and keep it there till the

sound has become rather taint. Then allow the pinched tube to open and notice that the sound

is now stronger on the left than the right and apparently located on the left. Cf. later

experiments on the location of tones.

On a. cf. Mach. op. cit. (under Ex. 89) p. 58. On b cf. Urbantschitsch. Zur Lehre von der

Schallempfindung, Pflüger's Archiv; XXIV. 1881, 574-579 and references then, given. See also

Stumpf, Tonpsychologie, I, 360-363, where other instances of fatigue an cited.

60. Inertia of the auditory apparatus. a. Inertia tending to keep the auditory apparatus out of

function can be demonstrated as follows. Place the ends of a rubber tube in the ears and set

upon the middle of it a low tuning-fork sounding as faintly as possible. Notice that the sound

does not reach its maximum intensity for an appreciable length of time; it the fork is barely

audible this may be as much as a second or two. Be careful not to increase the pressure of the

fork upon the tube after first setting it on; for that will produce an objective strengthening of the

tone; and allow an interval of several seconds between the tests so that the auditory apparatus

may again come completely to rest. A tuning-fork that will preserve these minimal vibrations for

some seconds and complete freedom from distracting noises will be found necessary for

success.

Cf. Urbantschitsch. Ueber das An- und Abklingen acustischer Empfindung,, Pflüger's Archiv,

XXV. 1881, 323.

61. Noise. Whether or not there is a distinctive sensation of noise different from that or a mass

of short, dissonant and irregularly changing tones is yet under debate, with something of the

weight of authority in favor of such a sensation. A little attention to the noises constantly

occurring, especially to their pitch, will easily convince the observer that a tonal element is

present. This is striking when resonators (cf. notes on apparatus for simultaneous tones) are

used, for they pick out and prolong somewhat the tones to which they correspond, but they are

not indispensable. On the other hand, attention to musical tones will often discover the

presence of accompanying noises.

Cf. Wundt, Physiologische Psychologie. I, 420; Stumpf. Tonpsychologie II, 497-514; also

Brücke op. cit. sub 69; Exner, Pflüger's Archiv XIII, 288 ff.; Mach, Analyse der Empfundungen,

Jena, 1886, 117.

62. Silence. When circumstances promise absence of external sounds, notice that many are

still present and distinct, though faintly heard. Notice also the pitch and changing character of

the subjective sounds to be heard. Our nearest approach to the sensation of absolute stillness

is this mass of faint inner and outer sensations.

Cf. Preyer, Ueber die Empfindung der Stille, Sammlung physiologischer Abhandlungen, Jena,

1877, pp. 67-72. This section on Silence is a portion of Preyer's study, Ueber die Grenzen der

Tonwahrnehmung. [p. 309]

SINGLE AND SUCCESSIVE TONES

Though musical terms are occasionally used in these experiments and some discrimination or

tones is necessary, it is believed that nothing is required beyond the average ability of the

unmusical.

Apparatus. The upper limit or pitch may be tested with the disk siren, with tuning-forks, with

steel cylinders and with little whistles of adjustable length. The last two instruments are most

commonly used and may be had from almost any dealer in physical apparatus, the cylinders

from $10 upward, the whistle (designed by Galton), at about $5.00. The Cambridge Scientific

Instrument Co., St. Tibb's Row, Cambridge, Eng., makes the whistle in two patterns, a simple

one at £1. 5s and a more elaborate one at £6. R. König, 27 Quai d'Anjou, Paris, also makes

two kinds, one at 12 the other at 20 fr., the latter probably a better instrument than that of the

Cambridge Scientific Instrument Co. at £1 5s. For description and prices of the more expensive

kinds of apparatus consult König's catalogue.

For testing the lower limit of pitch large tuning-forks may be used or the difference tones of

small tuning-forks or of stopped organ pipes. Large tuning-forks could probably be made well

enough for demonstrative purposes by almost any blacksmith and their pitch determined

approximately by making them record their vibrations graphically upon a piece of smoked glass

for ten seconds. The large fork with sliding weights (24-16 double vibrations per sec.)

manufactured for this purpose by König costs 300 fr. and his set of high pitched forks for the

difference tone method costs 340 fr.

For Ex. 65 and others that follow, almost any musical instrument or a considerable range of

pitch will answer.

For Ex. 65c. use a piston whistle such as is sold in toy stores at five cents. Two that are in the

laboratory here reach the proper pitch when their pistons are pushed in as far se possible. It

would be easy, if still higher tones were needed, to so alter the whistles as to make these

possible.

For Ex. 67 prepare a series of forks each differing from the next by about three vibrations a

second. Half a dozen C forks such as are sold at the music stores will answer, it those are

chosen that prolong their sound well. Take one of them as a standard and make the next

sharper by filing a little at the free end or the prongs till it beats (cf. Ex. 75) with the standard

three times in a second. Count for 10 seconds, if possible, and divide the total count by 10 to

find the rate per sec., counting the first beat nought. (For precautions to be used in attempting

an accurate count cf. Helmholtz, op. cit. 443 where Ellis gives all necessary particulars.) Having

brought this fork approximately to three beats per sec. take it as a standard and make the next

fork three beats sharper than it in the same way and so on. Make also a series of forks differing

by three vibrations each, which shall be flatter than the normal C. It will be well to make a

recount for greater accuracy after the forks have had a chance to cool. In order to flatten the

forks a little as mentioned in Ex. 67 little riders of rubber tubing may be placed upon the prongs.

Make these riders by cutting off quarter inch bits from tubing that will fit tightly upon the prongs

of the forks. For the resonance bottles mentioned in the same experiment take a four ounce

wide mouthed bottle and tune it to the forks by gradually closing its mouth with a bit of glass

(e.g. a microscope slide). When the amount of closure is found which gives the greatest

intensity of sound, fix the glass in position with wax. (For picture and description of such bottles

see Meyer, Sound, pp. 102-103). A standard instrument for giving such small differences of

pitch as are represented above by the sharpened forks is made by Anton Appunn of Hanau a.

M. under the name of a Tonmesser. (See picture and description in Wundt's Physiologische

Psychologie, I, [p. 310] 431 f.) and costs for the complete instrument, with pitches ranging from

32 to 1024 vibrations per sec. and a blowing table, 1060 marks. Single octaves without the

blowing apparatus range in price from 150 to 320 marks. The same maker also offers a set of

forks giving a series of tones differing each from the next by a small fraction of a vibration (Ton-

differenz-apparat) at 96 marks. See his price list also for other apparatus useful in the

experiments of this section.

For Ex. 69 a hydrogen generator (cf. an elementary chemistry) will be necessary. In blowing the

hydrogen and air bubbles it will be found convenient to have the mixed gases in one large

bottle and to force them out by pouring in water.

For Ex. 70 a pint bottle.

Some of the apparatus suggested at the beginning of the next section will also be found useful

in this.

63. Highest tones. With the apparatus at hand for the purpose, find what is the highest audible

tone; i.e. if the cylinders are used, the shortest cylinder which still gives a ringing sound on the

stroke of the hammer, or if the whistle is used, closest position of the plunger at which a tone

can still be beard beside the rush of sir. If a number of persons are tested it is not improbable

that some will yet hear the tone after it has become inaudible for the rest.

64. Lowest tones. a. If low pitched tuning-forks are at hand, find what is the slowest rate of

vibration that can yet be perceived as a tone. In some physiological laboratories electric tuning-

forks or interrupters are at hand which have vibration rates of 25 per second. Low tones can be

heard from these, though they have many overtones. The latter can be partly damped by

touching the tines mid-way of their length with the finger and partly avoided by bringing the ear

not to the free end but to a point somewhat further toward the handle. The determination of the

lower limit of audible pitch is difficult and uncertain because of the great difficulty which

observers, even those of trained ear, find in distinguishing these lowest tones from their next

higher octaves. b. The general character of these deep tones can be demonstrated with

sufficient clearness upon the contra octave (C

-C) of a church organ if one is accessible and

tuning forks are lacking.

65. Some characteristics of high and low tones. a. High tones are smoother than low tones.

This is clear with almost all tones used in music, and particularly so with those of reed

instruments. It Is largely due to the beating of the partial tones (see Exs. 82 ff. and 75 ff.)

among themselves and even with the fundamental tones. Play the scale of the instrument at

hand from the lowest to the highest, or sing the ascending scale. The difference of roughness is

observable also with simple tones, but only at lower pitches and is even there less marked. b.

High tones except the very high, produce a more intense sensation in proportion to their

physical intensity than do low tones. Strike a low tuning-fork in which the over-tones are to be

heard and notice that the over-tones can be heard at a greater distance than the proper tone of

the fork. c. Some high tones are particularly strengthened by the resonance of the outer

passage of the ear. These generally lie between c

and c

and give to the tones of this octave a

superior strength, and ear-piercing quality. They may be demonstrated easily with a small

piston whistle like that mentioned above. Find by adjustment of the piston the point at which the

tone is most piercing. Insert in the outer ends of the ear passages bits of rubber tubing half an

inch long (which will change the resonance of the passage, making them responsive to a lower

tone) and sound the whistle again. The piercing quality will be gone and the tone appear

decidedly weaker. Remove the bits of tubing and sound the whistle as before; the original

quality and intensity reappear. [p. 311] d. Very closely associated with the pure tonal sensations

are certain of a spatial quality. Compare in this respect the sensations of the tones observed in

Ex. c above, or better still those of Ex. 63, with those of Ex. 64 or any other deep tones. Play

the scale through the complete compass of any instrument, keeping this quality in mind. e. The

emotional shading of tones changes with their pitch. Recall the descriptive terms used: Deep,

low, tuneful, sharp, acute. Play the scale and judge of the appropriateness of these terms to

match the shades of feeling that mark the tones of low, middle and high pitch, distinguishing

those that refer to pitch from those enumerated in Ex. 86 which refer to timbre.

Cf. Stumpf. Tonpsychologie. I. 202-218, II. 56-59, 514 ff.; also Mach, op. cit. Under 61, p. 120

ff. On c and e cf. Helmholtz, Sensations of Tone (2nd Eng. ed.) p. 179 and p. 69. On d cf.

James, Psychology, II, 134 ff.

66. Recognition of absolute pitch. This experiment can, of course, give accurate results only

with those of very decided musical ear and skill, but It may be tried with any subject that knows

the names of the notes. a. Strike various notes in different parts of the scale of the instrument

and require the subject to name the note given. Record the note struck and the subject's

answer. He should be seated with his back toward the experimenter or should keep his eyes

closed.

Cf. Stumpf, op. cit. I, 305-313., also II, index, Höhenurteile, for experiments on trained

musicians.

67. Just observable difference in pitch. a. Test as follows with the set of mistuned forks

mentioned above. Let the subject pick out from the mistuned forks that which sounds to him

most like the normal fork, striking and holding them successively (never simultaneously) over a

resonance bottle. If all of them seem more than just observably different let him put the riders

(described above) on the one that is next higher and gradually lower he pitch by sliding them

toward the ends of the fork till the two, heard successively, are just different and no more. The

experimenter may then determine the error or the subject in vibrations per second

approximately by counting the number of beats produced by the forks when sounded together.

If the number of bests per second is less than 2 or more than 6 It will be best to get the

difference in pitch with some other of the forks first, so as to avoid too slow or too rapid

counting and from that to arrive at the difference from the standard fork. Repeat the test several

times and average the result, but take care to avoid fatigue. This experiment will not be refined

enough for testing those of keen musical ear.

Cf. Preyer. Grenzen der Tonwahrnehmung. Sammlung physiologischer Abhandlungen, I.

(Jena, 1877) 26 ff.; Stumpf. op. cit. I, 296-305; Luft. Wundt's Philos. Studien, IV. 514.

68. Differences in pitch that are just recognizable as higher or lower. It is easier to recognize a

difference than to tell its direction. a. Experiment as in 67a, but require the subject this time to

pick out and adjust a fork that is just observably sharper or flatter than the standard.

On a cf. Preyer op. cit. 28, 36. For experiments on extremely unmusical subjects cf. Stumpf,

op. cit. I: 313-335.

69. Number of vibrations necessary to produce a sensation of pitch. Arrange an apparatus for

blowing soap-bubbles with a mixture of hydrogen and air. Blow bubbles of different sizes and

touch them off with a match, either in the air, or, if proper precaution is taken to prevent the

ignition of the mixed gases in the vessel and any resonance in the pipe, while still The

explosion of these bubbles is supposed to produce a single sound wave. The pitch of the

sounds produced cannot be accurately given, but the report of the large bubbles is distinctly

deeper than that of the small ones.

Cf. Brücke, Ueber die Wahrnehmung der Geräusche, Wien. Sitzb., 3te Abth., XC, 1884, 199-

230. [p. 312]

70. The apparent pitch of tones is affected by their timbre, tones of dull and soft character

regularly seeming lower in pitch than those that are brighter and more incisive. Require the

subject to pick out on some stringed or reed instrument the tone corresponding to that

produced by blowing across the mouth of a medium sized bottle. Too low a note will generally

be chosen, at least by those without special musical training. The tones should be sounded

successively, not at the same time, during the test. Afterward they may be sounded together

and the pitch or the bottle determined approximately by finding with which tone of the

instrument its tone makes the slowest beats (cf. Ex. 75). It should be remembered, however

that it will be possible to get beats also with tones an octave lower and an octave higher than

that corresponding most nearly with the true pitch of the bottle tone. b. Repeat the experiment,

taking the pitch of the bottle first with the voice and then finding the tone on the instrument

corresponding to that sung. The illusion will probably disappear when the test is thus made.

Cf. Stumpf. op. cit. I. 176, 227-247, especially 235-245.

71. Recognition of musical intervals. Cause a familiar air to be played, first in the octave of c

and then in that of c" in the same or another key. Even those of no musical training will easily

recognize that the air (i.e. the succession of musical intervals in fixed rhythmical relations) is

the same in both cases; and any mistake or variation will be noticed as easily as if the air had

been repeated at the first pitch. The power of recognizing intervals is very much more highly

developed in persons of musical training, but any one that can whistle a tune at one pitch and

repeat it recognizably at another undoubtedly has the rudiments of it.

For exact methods of testing the accuracy of the power of recognizing intervals cf. Preyer

Ueber die Grenzen der Tonwahrnehmung, Jena 1876, pp. 38-64; and Schischmánow,

Untersuchungen über die Empfindlichkeit des Intervallsinnes, Wundt's Philos. Studien, V, 558-

600 and the references there given.

72. Pitch distance. Beside the interval relations of tones, and overshadowed by them in

musicians, are certain relations of separateness or distinctness or distance in pitch, which do

not depend on the ratios of vibration rates. Equal musical intervals (i.e. intervals between tones

that have vibration rates in a fixed ratio to each other, e.g. C D and c" d") do not correspond to

equal pitch distances. Sound the half tone interval cc-sharp[sic] through the range of the

instrument, beginning in the bass and ascending. Notice the increasing distinctness and

separation or the tones as the interval is taken higher and higher. For the very highest tones

there is probably a decrease of separateness again. The difference is most striking, however

with intervals smaller than those in common use, e.g. with quarter or eighth-tones. On the

harmonical[sic] (cf. notes on apparatus at the beginning of the next section) strike in succession

the c-sharp and d keys in the four lower octaves beginning with the lowest. In this instrument

the c-sharp key is given to another d, a comma, pr about one-ninth of a tone, flatter than the

regular d of the scale.

Cf. Stumpf, op. cit. I, 249-253; Lorenz, Untersuchungen über die Auffassung von Tondistanzen,

Wundt's Philos. Studien, VI, 1890, 26-103; also a prolonged discussion between Wundt and

Stumpf (and Engel) in succeeding numbers of the Studien and in the Zeitschrift für

Psychologie. Helmholtz, op. cit. pp. 264-65, 285, 287.

73. The effect of a given tone in a melody depends in part on the succession of tones in which it

stands. Cause a simple air, in which the same tone recurs in different successions of tones, to

be played and notice the difference in effect in the different circumstances.

Mach, op. cit. under 61, p. 130-131.

74. Tones that vary irregularly in time and in pitch are unpleasant. Test with a piston whistle. [p.

313]

SIMULTANEOUS TONES.

Apparatus. For the experiments of this section access to some large musical instrument is

essential, and a reed instrument is to be preferred to a piano if only one is to be used. A parlor

organ will answer in most cases, but sometimes the specially tuned Harmonical designed by

Ellis to illustrate the theories of Helmholtz (see description of the instrument in his translation of

Helmholtz's Sensations of Tone, pp. 466-469, also 17, 22 and 168), would be better. This

instrument is made as an aid to science by Messrs. Moore and Moore, 104-105 Bishopsgate

St., Within, London, E.C., at the very low price of from 8-5 to £10. For the proper tuning of the

instrument, however, a set of 19 forks is necessary costing £3-10 extra. In many of the

experiments a sonometer can take the place of a piano. A sonometer is simply a long hat box

with a very thin top which serves as a sounding board for the strings that are stretched over it.

One may be had from any physical instrument dealer at from $10.00 upward, or can be made

by a carpenter. For directions for making and dimensions see Mayer, Sound, (Appleton & Co.)

pp. 129-130.

For more perfect apparatus for the study or beats, difference tones, compound tones, timbre,

etc., consult the catalogues of König whose address has already been given, and of Anton

Appunn, Nürnbergerstrasse 12, Hanau, a. M. Germany. Both make resonators, those in

spherical form made by König are best and most expensive. A series of 10 corresponding to

the first ten partial tones of c (128 vibrations) costs 110 fr.; a series of 19, 170 fr. Appunn's in

conical form cost from 27 mk. for a set or 9 to 80 mk. for a set of 29.

The bottle whistles mentioned in Ex. 75 are

easily made by fitting a piece of rubber tubing

to the lip and neck of a bottle as in the cut, or

better still, by slitting the tube a little way so

that half the tube may extend an eighth or

three-sixteenths of an inch over the lip, but

care must be taken that it does not project too

far.

A pair of octave tuning forks will also be

needed. The large forks on resonance cases

(to be had of any physical instrument dealer

at a cost of from $5.00 to $20.00 according to

pitch) are much to be preferred, because they

sound longer after once being struck, but are

not indispensable. A pair of octave forks can

be made from an a' and a c" fork by filing the

a' till it gives c'. Choose an a' fork with thick

and heavy prongs and file it in the crotch and

along the lower half of the prongs inside, distributing the filing so as to leave the prongs of

equal thickness, till it begins to beat with c" when both are struck and have their stems pressed

against the table. Then continue the filing carefully till the beats can no longer be heard. The

filing warms the fork and makes It a little flatter than when cold; this of course must be taken

into account. To make a c''' fork, if one is desired, a c" should be used and the cutting must be

at the free end of the prongs. In one made here about three-quarters of an inch was taken off.

The tuning is as before by filing until the beats with c" are first heard, then grow slower and

finally disappear. In the same way an a" may be made as the octave of a', but these small forks

do not vibrate very long. [p. 314]

75. Beats. When tones not too greatly different in pitch are sounded at the same time they

mutually interfere and mage the total sensation at one instant more intense and the next instant

less intense. This regular variation in intensity is called "beating." Exs. 67 and 70, where beats

have been need incidentally, are a sufficient introduction to them. a. The rapidity or beats

depends on the difference in the vibration rates or the beating tones. Prepare two bottle

whistles of the same size and blow both at the same time. Slow beats will probably be heard. If

not, pour a little water into one bottle (thus raising the pitch of its tone) and blow as before.

Continue adding water a little a time till the beats lose themselves in the general roughness of

the tone. Blow the bottles separately now and then to observe the increasing difference in pitch.

The same may be shown with a couple of piston whistles, if they are first adjusted to unison

and then the piston or one or the other pushed in or pulled out. b. The rate at which the

roughness of rapid beats disappears, as also the rate which produces the greatest roughness,

differs with the pitch of the beating tones. Sound the following pairs of tones which have

somewhat near the same difference in vibration rates per sec., namely, 33; and observe that

the roughness from the beats decreases and finally disappears entirely at about the fourth pair:

b' c", c' d', e g, c e, G c, C G. The a' and c" tuning forks give a vanish of roughness,

representing a rate of 80-88 per sec.

Cf. Helmholtz, op. cit., pp. 159-173; Stumpf. II, 449·497, especially 461-465.

76. Beats betray the presence of very faint tones both because the total stimulus is actually

stronger in the phase of increased intensity and because intermittent sensations are

themselves more effective than continuous ones. a. Strike a pair of beating tuning forks and

hold one at such a distance from the ear that it is very faint or quite inaudible. Then bring the

other fork gradually toward the ear and notice the unmistakable beats. b. Strike a tuning-fork

and hold it at a distance as in a, being careful to have the fork sidewise or edgewise, not

cornering, toward the ear. Rotate the fork one way and the other about its long axis and

observe the greater distinctness or the tone, due in this case simply to its intermittence.

77. Beats are in general attributed to the tone that is attended to; in the absence of otherwise

determining causes, to the louder tone, if there is a difference in intensity, to the lower tone, or

to the whole mass of an unanalyzed compound tone (see introduction to Ex. 82). a. Set two

properly tuned resonance bottles about a foot apart on the table. Strike two forks that beat and

hold them over the bottles. While both are about equally intense it is easy by mere direction of

the attention to make the beats shift from one to the other. b. Turn one of the forks about an

eighth of a turn about its long axis, which will weaken its tone and observe that the beats seem

to come from the other fork. By moving first one fork and then the other the location of the

beats may again be made to shift at pleasure. c. Warm the c' fork in any convenient way,

(holding it clasped in the hand will do.) This will flatten it somewhat. Strike it and the c" fork and

press the stems of both on the table at the same time, or better on the sounding board of the

sonometer. Observe that the bests seem to come from c' fork unless it is very faint. d. Tune a

string of the sonometer so that its third partial (or corresponding harmonic) beats slowly with

the c'' fork. (On partials and harmonics cf. Exs. 82-85.) Strike the tuning fork and hold it over a

resonance bottle, or press its stem against the table at arm's length from the string. Then pluck

the string and attend to its tone, the beats may seem to affect the whole compound tone of the

string. But this will not happen if the tone of the string is analyzed or if the attention is directed

to the fork. The same may be tried on the piano [p. 315] picking out from the mistuned c" forks

one that beats slowly with c" on the piano. Strike the f key and hold it down; strike the fork and

observe the bests as before.

Cf. Stumpf, op. cit. II. pp. 489-497.

78. Difference tones.[2] When two tones are loudly sounded at the same time there results

(probably from supplementary vibration of the tympanic membrane and ear bones) a third tone

of a pitch represented by the difference of the vibration rates of the two original or generating

tones. These difference tones are easy to hear when they lie considerably deeper than the

generators, when the latter are loud and sustained, and when they make a consonant interval,

though the latter is not essential. A loud difference tone may itself take the part of a generator

and produce yet another difference tone -- a difference tone of the second order -- and so on,

though difference tones of higher orders are heard with difficulty even by skilled observers of

trained ear. Difference tones are hard to hear on the piano and similar stringed instruments,

because of the rapid decline in the strength of the generators. a. Repeat Ex. 75a continuing to

pour water into one of the bottles till the difference tone appears. At first the roughness of the

beats and the difference tone may both be heard at once. Try the same with the piston whistles,

first setting them at unison and then slowly pushing the piston of one in or out while blowing

hard. The beats will almost immediately give place to a low difference tone which may be heard

ascending through several octaves before becoming indistinguishable from the generators. The

double warning whistles used by bicyclists give a fine difference tone, to which indeed they owe

their deep and locomotive like quality. b. Difference tones are strong on reed instruments.

Press the adjacent white keys of a parlor organ, or the harmonical, by twos, beginning at c and

going up a couple of octaves. If there is difficulty in hearing the difference tone, sound the

upper tone intermittently and listen for the difference tone at the instant of pressing the key. c.

Sound c" and d" which should give C for a difference tone (594-528=66). Sound also d" and e"

which should give the same (660-694=66). If, however, the tuning is inexact, as it is

intentionally in the tempered tuning of keyed instruments, these difference tones will be

somewhat different and may be heard to beat with one each other when c", d" and e" are

sounded at once. Notice that you do not get these beats when the tones are sounded in pairs.

On the harmonical this difference may be brought about by sounding one of the tones flat by

pressing its key only a little way down. The same thing may be shown with three piston whistles

blown at once, by a little careful adjustment of the pistons. d. The location of difference tones.

The location of these tones is sometimes influenced by the location of their generators, but

under favorable circumstances they seem to arise in the ears or even in the head. This is

strikingly the case, both for the blower and the listeners, with the difference tones produced

with the piston whistles.

Cf. Helmholtz op. cit., pp. 152-159; Stumpf, op. cit., II, pp. 243-257. König, Quelques

expériences d'acoustique, Paris, 1882.

79. Blending of tones. The degree to which tones blend with one another differs with the

interval relation of the tones taken. It is, according to Stumpf, greatest with the octave, less with

the fifth, less again with the fourth, alight with the thirds and sixths and least of all with the

remaining intervals. Try on the instrument the extent to which the tones forming these intervals

blend, also those forming intervals greater than an octave: double octave, twelfth, etc, b. The

blending in [p. 316] case of the octave is so complete under favorable circumstances as to

escape the analysis of trained ears. Use two tuning-forks, one an octave higher than the other,

on resonance cases or held over resonance bottles. Sound the forks, first the higher, then the

lower. For a while the higher fork will be heard sounding in its proper tone, but by degrees it will

become completely lost in the lower and a subject with closed eyes will be unable to say

whether or not it yet sounds. Stop the lower fork or remove it from its resonance bottle and

notice that the higher is still sounding. Notice the change in timbre (cf. Ex. 86) produced by the

stopping of the higher fork --something like the change from the vowel O to the vowel U (oo).

On a cf. Stumpf, op. cit. II, pp. 127·218, especially 135-142, 353; for his experiments on the

unmusical confirming his grades of blending cf. 142-173. On b cf. Stumpf, op. cit. II. pp. 352-

356, and Helmholtz, op. cit. pp. 60-61.

80. Analysis of groups of simultaneous tones. Ease of analysis depends on a number of

conditions, among others on the following. a. Analysis is easier for tones far distant in the scale.

Compare the ease of recognizing the sound of the c" fork when c' and c" are sounded together,

with that of recognizing c''' when sounded with c'. Compare also the ease of distinguishing c'

and a' with that of distinguishing c' and a". b. Analysis is made easier by loudness in the tone to

be separated. Repeat Ex. 79b sounding the c' faintly c" strongly. No difficulty will be found in

keeping the latter distinct. c. Analysis is easier when the tones make intervals with little

tendency to blend. Compare the ease of analysis or c' c" and c' a' or a' c". Also notice that the

addition or d" (octave of d' fifth of g' fourth below g") to the chord g d' g' g" produces a lees

striking change than the addition of b' (major third of g' minor sixth below g"). d. Analysis is

easier with sustained than with short chords. Repeat the last experiment making the chords

very short and notice that the difference made by inserting either d" or b' is less marked. Cf.

also Ex. 95.

Cf. Stumpf, op. cit.. II, 318-361, also his experiments. 362-382.

81. The lower tone of a chord fixes the apparent pitch of the whole. a. Repeat Ex. 79b and

notice that when the c' fork is stopped the tone appears to jump upward an octave in pitch (i.e.

it takes the pitch of the c" still sounding); but when the c" fork is removed the quality of the tone

is changed but not its pitch, b. Strike the chord C c" e" g" or G e' g' c" and compare the effect

upon the pitch of the whole mass of tone produced by omitting C or G alone with that of

omitting any one or all three of the higher tones. See also the function of the lowest partial of a

compound tone in fixing the pitch, noticed below.

Cf. Stumpf, op. cit. II, 382-392.

Compound tones which on casual hearing seem single tones but in reality are chords deserve

special attention. The tone given by the C string of a piano is made up of at least C, c, g, c', e'

and g' and generally other tones. The lowest tone of the group gives the pitch attributed to the

whole and is known as the fundamental, the other tones as over-tones. In another way of

naming them, the component tones are all partial tones or partials, the fundamental being

called the first or prime partial, the next higher the second partial and so on. It should be

observed that the first over-tone is the second partial tone, the second over-tone the third

partial, and in general that the same tone receives as a partial tone a number one higher than

se an over-tone. The vibration rates of the partial tones of a compound are generally once,

twice, three times, four times, the rate of the fundamental, and so on. In some cases, however

e.g. in bells and tuning-forks one or more of the partial tones may have vibration rates not

represented in this series and discordant with the [p. 317] fundamental tone. In what follows,

the regular series or partial tones is meant except where the contrary is specified.

On the physics and physiology of this matter and others treated in this and the preceding

section cf. Tyndall, On Sound; Blaserna, Theory of Sound in its Relation to Music; Taylor,

Sound and Music; Helmholtz, Sensations of Tone. The last is of course the great classic on all

such matters; the next to the last is very simple and untechnical and perhaps the best for those

approaching the subject for the first time.

82·. Partial tones: Analysis with resonators. If resonators are at hand the demonstration of the

partial tones will be easy. Sound on the instrument the tones to which the resonators are tuned,

and notice that they resound strongly to these tones and less strongly or not at all to other

tones adjacent in pitch. Then sound the tone to which the largest of the resonators is tuned,

and try the rest of the resonators in succession. Notice that others also resound (at their own

proper pitch), thus betraying the presence or the tones to which they are tuned, and thus the

composite character of the tone under examination. Which resonators will "speak" will depend

on the instrument used, reed instruments giving a long and perfect series, pianos and stretched

wires a perfect series generally as far as the 9th or 10th partial, and stopped organ pipes a

short series. If difficulty is found in knowing when the resonator is resounding it will be found

useful to apply it to the ear intermittently, alternating, for example, two seconds of application

with two seconds of withdrawal.

83. Partial tones: Analysis by indirect means. a. By sympathetic vibration. This succeeds

especially well with the piano. Press the c key and hold it down so as to leave its strings free to

vibrate then strike the C key forcibly and after s couple of seconds release it. The c strings will

be found to be sounding. Repeat, trying c-sharp or b instead of c they will be found not to

respond. Repeat the experiment, substituting g, c' e' g', and c"; all will be found to respond but

in lessening degrees. Other keys between C and c" may be tried but will be found in very faint

vibration if at all. b. By beats. This will succeed best with a reed instrument, e.g., a parlor organ

or the harmonical. By pressing the keys of the instrument only a little way down any of its tones

may be sounded a little flatter than its true pitch and so in condition to beat with any other tone

having that true pitch. Sound at this flattened pitch the over-tones of C in succession while C is

sounding, and notice the slow beets that result. For verification sound other tones not over-

tones of C and notice that the beats when present are much more rapid.

84. Partial tones: Direct analysis without special apparatus. The directions given here apply to

the sonometer but will be readily adaptable to any stringed instrument in which the strings can

be exposed. It is easier to hear any partial tone in the compound, if the partial is first heard by

itself and then immediately in combination with the rest. On strings this is easily done by

sounding the partials as "harmonics." Pluck the string near one end (say about one-seventh of

the length of the string from the end), and immediately touch it in the middle with the finger or a

camel's-hair brush. The fundamental will cease to sound and its octave (the second partial) will

be left sounding, as an "harmonic." With it sound also other even-numbered partials, but less

strongly. Pluck as before and touch the string at one-third its length; the third partial will now

sound out strongest, with the sixth ninth, etc., more faintly. Thus by plucking the string and

touching it respectively at one-half, one-third, one-fourth, one-fifth, one-sixth, one-seventh, one-

eighth, one-ninth and one-tenth its length from the end, the series of tones corresponding to the

2d, 3d, 4th, 5th, 6th, 7th, 8th, 9th and 10th partials can be heard, each in large measure by

itself. In getting the higher "harmonics " it will be found better to pluck nearer [p. 318] the end

than one-seventh, end in no case should the string be plucked at the point at which it is

presently to be touched. (cf. Ex. 88b.) To hear the partial tones when sounding; in the

compound, proceed as follows. Sound the required tone as an "harmonic," and then keeping

the attention fixed on that tone, stop the string and pluck it again, this time letting it vibrate

freely. The tone just heard as an "harmonic" will now be heard sounding with the rest as a

partial. When the partial is thus made out, verify the analysis by touching the string again and

letting the tone sound once more as an "harmonic." Try in this way for the partials up to the

tenth; first for the 3d, 5th and 7th, afterward for the 6th, 4th and the 2d, which is the most

difficult of all. It has been said that analysis is easier at night, (not alone on account of the

greater stillness) when one ear is used, and that certain positions of the head favor certain

partials.

85. Partial tones: Direct analysis without apparatus. Certain parts of a compound tone are

sometimes so separated by their dissonance, intensity or pitch that they stand out with striking

clearness. a· Strike a tuning fork on a hard surface and observe the high, ringing, dissonant

partials. They fade out before the proper tone of the fork, and are heard best when the fork is

not held near the ear. b. As the tone of a string is allowed to die sway of itself, different partial

tones successively come into prominence. Try with a low piano string, keeping the key pressed

down while the sound fades, or on the sonometer. Something of the same kind, but less

marked, happens in the dying sway of a low tone on a reed instrument when the sir is allowed

to run low in the bellows. c. When a tone is sounded continuously for some time, for example,

on a reed instrument with one of the keys clamped down, different partials come successively

into prominence, either through varying fatigue or the wandering of attention.

Cf. Helmholtz op. cit. pp. 36-65; Stumpf, op. cit. II, 231-243, see also the index under

Obertöne.

86. Timbre. The peculiar differences in quality of tones, (distinct from pitch and intensity) which

are known as differences in timbre (tone color, clang tint, Klangfarbe), are due to differences in

the number, pitch and intensity of the partial tones present. Compare in this respect the dull-

sounding bottle tones or the tones of tuning forks held over resonance bottles and the more

brilliant tones of a reed or stringed instrument; the first are nearly simple tones, while the

second have strong and numerous over-tones. a Notice the difference in quality between the

tone given by a tuning fork held before the ear and that given by the same fork when its stem is

pressed upon the table. In the second position the over-tones are relatively stronger. b. Notice

the differences in quality in the tone of a string when it is plucked in the middle, at one-third its

length and at about one-seventh. When plucked in the middle, many odd-numbered partials are

present and the even-numbered partials are either absent or extremely faint, and the tone is

hollow and nasal; when plucked at one-third, the third, sixth and ninth partials are wanting and

the tone is hollow, but not so much so as before; when plucked at one-seventh all the partials

up to the seventh are present (for their theoretical intensities cf. Helmholtz op. cit. p.79). c. Try

also plucking very near one end, plucking with the finger-nail and striking the string with a hard

body, e.g., the back or a knife blade; all these bring out the higher and mutually discordant

partials strongly and produce a brassy timbre. Cf. also Ex. 79b.

Cf. Helmholtz op. cit., pp. 65-119; Stumpf, op. cit., II, 514-549.

87. In successive chords the whole mass or tone seems to move in the same direction as the

part that changes most. Strike in succession the chords e' g'-sharp b' e", a a' c"-sharp e" or a c'

e' c", a c' f' c". If the attention is directed to the bass in the first example and to the alto [p. 319]

in the second the whole mass of tone will appear to descend in the first case and to ascend in

the second. If the attention is kept on the soprano part the illusion will not appear, as also when

the observer examines his sensations critically. Cf. also Ex. 77d where beats of a partial tone

are attributed to the whole compound tone.

Cf. Mach, Analyse der Empfindungen, 1886, 126-127; Stumpf, op. cit., II. 393-395.

88. Simultaneous tones interfere somewhat with one another in intensity.

a. Play the groups of notes numbered 1, 2 and 3 and observe the slight increase in the

apparent intensity of the remaining tones as one after another drops out, making 1 sound like

la, 2 like 2a, and so on. On the piano it will be well to play the notes an octave or two deeper

than they are written.

b. Play the notes marked d and notice that the increase of loudness seems to affect the note

(highest or lowest) that receives particular attention, making the effect in one case like 4a, in

the other like 4b.

Cf. Mach, op. cit. 126; Stumpf, op. cit., II, 418-423.

89. Consonant and dissonant intervals. a. The consonant intervals within the octave are the

unison, octave, fifth, fourth, major sixth, major third, minor third, minor sixth. They will be round

to decrease in smoothness about in the order given. Try them beginning with the octave and at

c, as follows: c c', c g, c f, c a, c e, c e-flat, c a-flat. Try the last four intervals also in the octave

of c" or c"' and notice that they are less rough than when taken in the octave of c. Any other

intervals within the octave are dissonant. Try c c-sharp, cd, cb, cb-flat, cf-sharp. The roughness

is due to beating partial tones and in general is greater when these stand low in the series and

are loud, and when they lie within a half-tone of each other. Work out for the tones of several of

the intervals the series of partial tones up to the eighth. In general the extension of intervals into

the second octave (taking the higher tone an octave higher or the lower tone an octave lower)

does not change the fact of consonance or dissonance, though it may change the relative

roughness. b. Those fitted by musical training to pronounce upon questions of consonance and

dissonance hold that dissonance can be perceived between simple tones under conditions that

exclude beats, and that consonance is not simply the smooth flowing of tones undisturbed by

beats. The test is easy to make -- simply to hold tuning forks making the intervals to be tested

one before each ear, and if there are beats to carry the forks far enough away in each direction

to make the beats inaudible -- but only those of musical ear can pronounce upon the result.

Cf. on a, Helmholtz, op. cit., pp. 179-197. Stumpf, op. cit. II, 470, 460. Wundt, Physiologische

Psychologie, I, 439, II, 47 ff.; Mach, op. cit., 129-130. [p. 320]

90. Consonant and dissonant chords. In order to form a consonant chord all the Intervals

between the tones used must also be consonant. The only chords of three tones which fulfil this

condition within the octave are represented by the following: Major c e g, c f a, c e-flat a-flat,

minor c e-flat g, c f a-flat, c e a. Try these and for comparison any other chord of three tones

having c for Its lowest tone.

Cf. Helmholtz, op. cit. p. 211 ff.; Wundt, op. cit. II, 61, 67 ff.

91. Major and minor chords. Compare the chords c" e'' g" and c" e"-flat g". This unmistakable

difference in effect depends in part at least on the fact that in the major chord the difference

tones of the first order are lower octaves of c" itself, while in the minor chord one difference

tone is not such at all and if taken in the same octave with the chord would be highly dissonsnt.

For the major chord, when taken in the octave of c", the difference tones are c and c", for the

minor chord c, e-flat, A-flat. Try on a reed instrument the difference tones generated by c" e", e"

g", c" e"-flat, e"-flat g", first separately; and then while c" and g" are kept sounding, strike e" and

e"-flat alternately.

92. Cadences. Modern music requires the prominence of the key note or tonic and of the chord

in which it holds the chief place at the beginning of a piece of music and at the end. The feeling

of the appropriateness of this close and especially of the succession of chords in the following

cadences can hardly fail to appeal even to the nonmusical.

Cf. Helmholtz op. cit., 293.

BINAURAL AUDITION AND THE LOCATION OF SOUNDS.

Apparatus. In addition to apparatus already used, a pair of unison tuning-forks on resonance

cases will be needed in Ex. 98d, (and in several of the other experiments such large forks,

unscrewed from their cases, are almost indispensable, because the tones of ordinary small

forks are too faint and last too short a time), also a mechanical telegraphic "snapper-sounder,"

a yard-stick and a retort stand. The "snapper sounder,"[sic] common as a toy a few years ago,

can be bought of E.S. Greeley & Co., 5 & 7 Dey St., New York, at from 30 to 75 cents.

93. Unison tones heard with the two ears. a. Strike a pair of unison forks that will sound equally

loud and vibrate an equal length of time, and hold one before each ear, three or four inches

away; a single tone of rather indefinite location will be heard. As the forks are brought nearer,

their tone seems to draw by degrees toward the median plane; and when they are very loud

and near, the tone may seem to be in the head. Return the forks to their first position and then

move one a little nearer or a little farther away, and notice that the sound moves to the side of

the nearer fork. When the difference in distance has become considerable that fork alone will

be heard. b. Bring the forks again into the positions last mentioned -- one near and one far, (or

better, place one fork on a rubber tube one end of which has been inserted in [p. 321] the

opening: of the ear and hold the other fork before the other ear) and then with the free or more

distant fork make slow rythmical[sic] motions toward and away from the ear, or rotate the fork

slowly about its long axis, attending meantime to the fork on the other side. Alternate variations

in the intensity of the tone of this fork corresponding to the approach and recession oft he other

and apparently unheard fork can be heard. c. Repeat b and notice that when the changes in

intensity are considerable there is a simultaneous shitting of the place of the tone, toward the

median plane when the tone grows stronger and away when it grows fainter. These changes of

place are, however, less marked than changes in intensity and those accompanying slight

changes in intensity generally escape observation.

Cf. Schaefer, Zur interaurealen Lolalisation diotischer Wahrnehmungen, Zeitschrift für

Psychologie I, 1890, 300-309; also Silvanus P. Thompson, On Binaural Audition, Phil. Mag.

Series 5, IV (July-Dec., 1877) 274-276; VI (July-Dec., 1878) 383-391. XII (July-Dec., 1881)

351-355; On the Function of the two Ears in the Perception of Space, XIIl (Jan.-June, 1882)

406-416; and the references given by these two authors.

94. Bests heard with the two ears. a. Operate as in Ex. 93a, with forks beating three or tour

times a second. b. Try with a pair of very slow beating forks (once in two or three seconds).

Notice a shifting of the sound from ear to ear corresponding to the rate or beating. c. Try again

with a pair of rapid beating forks (twenty or thirty a second) and notice that the beats are heard

in both ears.

Schaefer, op. cit. also Ueber die Wahrnehmung und Localisation von Schwebungen und

Differeztönen. Zeit.f. Psy. I, 1880, 81-98.

95. Difference of location helps in the analysis or simultaneous tones. Compare the ease with

which the tones of a pair of octave forks are distinguished when the forks are held on opposite

sides of the head with the difficulty of analysis in Ex. 79b.

Cf. Stumpf, op. cit. II, 336, 363.

96. Judgments of the direction of sounds. These depend in general on the relative intensity of

the sounds reaching the two ears, but there is pretty good reason to believe that other things

cooperate and that tolerably correct judgments, both as to distance and direction, can

sometimes be made from the sensations of one ear. a. Let the subject be seated with closed

eyes. Snap the telegraph snapper at different points in space a foot or two distant from his

head, being very careful not to betray its position in any way, and require him to indicate the

direction of the sound. Try points both in and out of the median plane. Observe that the subject

seldom or never confuses right and left but often makes gross errors in other directions.

Constant tendencies to certain locations are by no means uncommon. b. Have the subject hold

his hands against the sides of his head like another pair of ears, hollow backward, and try the

effect upon his judgment of the direction of the snapper. c. Find approximately how far the

snapper must be moved vertically from the following points in order to make a just observable

change in location: on a level with the ears in the median plane two feet in front; opposite one

ear, same distance; in the median plane behind the head, same distance. Find the just

observable horizontal displacements at the same points. A convenient way of measuring these

distances is to clamp a yard-stick to a retort-stand, bring it into the line along which

measurements are to be made and hold the snapper over the divisions of the stick. Snap once

at the point of departure, then at a point a little way distant in the direction to be studied; again

at the first point, so that the subject may keep it in mind, and then at a point a little more distant,

and so on till a point is finally found which the subject recognize as just observably different.

Repeat, alternating snaps at the point of departure with those at a greater distance than that

Just found, decreasing the latter till a point is found where the directions can be no longer

distinguished. Make a number of tests each way and take their average. [p. 322] d. Continuous

simple tones are very difficult to locate. Place a tuning-fork on its resonance case at some

distance in treat of the subject (seated with closed eyes) another at an equal distance behind

him. With the help of an assistant strike both forks and after a little have one of them stopped

and the mouth of its resonance box covered. Require the subject to say which has been

stopped. His errors will be very frequent. Compare with this his ability to distinguish whether a

speaker is before or behind him.

Cf. on a Preyer Die Wahrnehmung der Schallrichtung mittelst der Bogepgänge, Pflüger's

Archiv, 1887; also v. Kries, Ueber das Erkennen der Schallrichtung, Zeit. F. Psy. I, 1890. 235-

251. On c cf. Münsterberg, Raumsinn das Ohres, Beiträge zur experimentellen Psychologie,

Heft, II, 1889. Rayleigh. Nature, XIV, 1876, 32.

97. Intercranial[sic] location of sounds. a. Sounds originating outside the head are not located in

the head when heard with one ear. Hold a loud sounding tuning fork near the ear or place it on

a rubber tube, one end of which is inserted in the opening of the ear, and notice that the sound

when strong may be located in the ear bat does not penetrate further. Insert the other end of

the tube in the opening of the other ear and repeat. The tone if loud will appear to come from

the inside of the head. Removing and replacing the fork several times will help to give

definiteness to the location. b. Repeat the experiment, but use a fork sounding as faintly as

possible (e.g. set in vibration by blowing smartly against it), and notice that the location when a

single ear receives the sound is not so clearly in the ear, and, when both receive it, not so

clearly in the head, perhaps even outside of it. Cf. also Ex. 98b. These experiments may also

be made with beating tones instead of a single one.

Cf. Schaefer, op. cit. under 94.

98. Location of the tones of tuning-forks pressed against the head. a. Strike a large and loud

sounding tuning fork and press its stem against the vertex. The tone will seem to come from the

interior of the head chiefly from the back. While the fork is in the same position close one of the

ears, not pressing it too tight; the sound will immediately seem to concentrate in the closed ear.

Have an assistant manage the fork and close the ears alternately. Something of the same kind

happens when a deep note is sung; close first one ear and than both and notice the passage of

the tone from the throat to the ear and finally to the middle of the head. b. Have an assistant

manage the fork and close both ears. Notice that when fork is pressed on so as to make the

tone loud the intercranial location is exact, but when the pressure is relaxed and the tone is

faint the location tends to be extracranial. c. Try setting the fork on other places than the vertex.

Notice that in the occipital and parietal regions the sound appears in the opposite ear. d. Take a

long pencil in the teeth like a bit and rest the stem of a tuning-fork vertically on it near one end

and close the ear on the other side; the sound will seem to be located in the closed ear. Then

gradually tilt the fork backward toward a horizontal position, keeping if in contact with the pencil

till its tip is opposite the open ear. The tone will change its place from the closed to the open

ear.

On a and b cf. Schaefer, op. cit. under 94; on c cf. Thompson, second article referred to under

94.

Footnotes

[1] Whether this essence is of the same strength as that used by Lombroso and Ottolenghi in

their experiments to which reference is made below, the writer does not know.

[2] König distinguishes between "difference tones" and "beat tones." Both tones, however,

generally have the same pitch and the older term for them has here been retained; Strictly

speaking, however, the "difference tones" heard in these experiments are "beat tones."

A LABORATORY COURSE IN PHYSIOLOGICAL PSYCHOLOGY.

By Edmund C. Sanford (1892)·

(Third Paper [pp. 474-490].)

V. -- VISION.

THE MECHANISM OF THE EYE, AND VISION IN GENERAL.

Apparatus. Many oft he experiments of this section can be performed with very simple

apparatus, made on the spot. The following materials will be needed : Pins, cards, corks, a

candle, a couple of postage-stamps, a watch glass, pieces or colored glass, black and white

card-board (not shiny), colored papers, a light wooden rod. Four inches square is a convenient

size for the glass, of which two pieces should be cobalt blue, one red. Any colored papers will

serve; those made for artificial flowers are easy to get in large variety of tints. A fine series of

papers in Helmholtzian colors is sold by R. Jung, Heidelberg. In addition to these supplies there

is need of a double convex lens short focus, two inches or more in diameter; an ordinary

burning or reading glass would do, though those mounted on an adjustable stand, costing

$2.50 and upward from the physical instrument dealers, are more convenient; also a concave

spectacle lane.

For Ex. 99 a pink-eyed rabbit and a little modeling clay are necessary. An instrument for

facilating[sic] Ex. 103 (a Phakoscope) can be had from Jung for 25 marks; a more elaborate

instrument of the same name is quoted by the Cambridge Scientific Instrument Co., St. Tibb's

Row, Cambridge, England, for £8-8.

For Ex. 109 and other experiments a firm

head rest of some sort is required. For most

purposes one like that shown in the cut will

answer well enough and can easily be made.

Fig. A shows a board about 20 in. high and l2

in. wide with a U-shaped opening cut in the

top to receive the lace, the chin resting at a.

Fig. B, the top view, shows the cross piece

against which the forehead rests at b. The

whole when in use is clamped to the edge or

the table. When a complete immobility of the

head is desired it is best secured by providing

a thin board cut out so that it can be put into

the mouth and taken between the jaws. If the

parts open which the teeth rest are covered

with sealing wax and are bitten upon while

the wax is still soft, not only is a firm support

for the head seemed, but the head can be

returned again exactly to its former position

after an interval, if desired. Such a mouth board could [p. 475] easily be added to the support

shown in the out. For pictures of such mouth boards cf. Hermann, Handbuch der Physiol. III, pt.

2, pp. 440, 473 and 478, also Helmholtz, Optique physiologique, p. 665 (p. 517 of the first

edition) Aubert, Physiologische Optik, p. 647.

For Ex. 110 make a saturated solution of chrome-alum in water, filter and put into a flat-sided

clear glass bottle. Dilute, if necessary, till the yellow spot can be observed as described in the

experiment.

Ex. 115 requires a pair of electrodes and a

battery. The electrodes can be made by

soldering connecting wires to plates or brass

or zinc, two and a half inches wide by three

long and covering them with cloth. Some kind

of a key for opening and closing the circuit

and a commutator for changing the direction

of the current are helpful, though not

essential. Any battery giving a sufficiently

strong current will do; one of four cells of the

"gonda" pattern has proved sufficient for

demonstrative purposes, and every much

weaker one will serve for showing the flash

from electrical stimulation.

Ex. 119, involves a rotation apparatus of

some kind and a disk traced with a spiral as

in the cut. Any rotation apparatus will do, but

if the laboratory is supplied with batteries one of the small electric motors now to be had at a

very low price is easily adaptable for use and is extremely convenient.[1] A Porter motor,

retailing at $3.00, has been used with success In this laboratory. It is well to have the disk

large, a foot in diameter, and the line of the spiral thick, three eighths of an inch across, and a

good black.

In a number of experiments black or white screens are to be used. A simple piece of black or

white card-board will generally answer but sometimes the more permanent form indicated in

the cut is convenient. It consists of an upright board 18 inches high, seven eights[sic] of an inch

thick and 12 Inches wide, firmly fixed on a wooden base. In the cut the base in made too large.

One side of the upright is covered with black card-board (or painted a dull black), the other with

white card-board.

Of models helpful in understanding the

mechanism and functions of the eye there are a

number. Anatomical models are quoted, among

others, by Jung (12 marks); by Kny & Co., 17

Park Place, New York, ($5.00 to $28.00); by

Queen & Co., 924 Chestnut St., Philadelphia

(Auzoux models, $19.00 and $20.00 without

duty). Of physiological models, the best for

accommodation and the like is Kühne's optical [p.

476] eye made by Jung, at 65 marks, by the

Cambridge Scientific Instrument Co., at £7. The

action of the muscles and the behavior or the eye

in motion is illustrated by the Ophthalmotrope,

described with cut by Helmholtz, Optique

physiologique p. 678, (p. 527 in the German

edition).This instrument is to be had of Jung, at

25 marks. Of the Cambridge Scientific Instrument

Co., for £10; and of other dealers also. Another

instrument for the same purpose, called the Blemmatotrope is described by Hermann in Pflüger's

Archiv, VIII, 1873, p 305. The motions of the eye and their effect on the retinal image, such

especially as those mentioned in Ex. 123, are finely shown by the Phenophthalmotrope of

Donders, described in v. Graefe's Archiv für Ophthalmologie, Bd. XVI, 1870, and Bold by Jung at

30 marks. An improved form of the instrument is to be had of D.B. Kagenaar, Rijks-Universiteit,

Utrecht, at 40 guilders. Suggestions for simple illustrative apparatus will be found with the

description of the experiments.

Standards and rods with clamps and universal joints, thought not distinctively for visual

experiments, are by far the most important of the general conveniences of a laboratory. They

enter into the setting up of very many experiments and a liberal share of even a small

appropriation may well be invested in them. Ordinary clamps can be bought in all sizes at the

hardware stores at prices from ten cents upward. The standards and couplers to be had from

the chemical and physical instrument dealers are made for another purpose and are not very

satisfactory in the psychological laboratory. Those made for physiologists and photographers

are better. Wilhelm Petzoldt, Bairische Str. 13, Leipsig[sic], makes a considerable variety of

which the following have been found useful in the physiological and psychological laboratories

of Clark University. Standards: simple tripods with interchangeable rods of 9 and 13 mm.

diameter, 6.50 marks, and large tripods with leveling screws In two of the feet and carrying two

of the above mentioned rods, at the same time, 16 marks. Table-clamps, which screw on to the

edge of the table and are bored to receive the rode, thus taking the place of tripods: two kinds,

one bored for the 9 mm. rods, but having only a vertical hole, 2.75 marks; the other bored for l3

mm. rods having both horizontal and vertical holes, 3.50 marks. Couplers to fit both sizes of

rods: those for the 13 mm. rod (of iron) and connecting the rods only at right angles, 2 marks,

those for the 9 mm, rods (of brass) and connecting the rods either at right angles or parallel

2.75 marks. Petzoldt also makes smell clamps of various sizes, like those furnished with the

chemical sets, mounted upon the 9 mm. rods, at 3 marks. The advantage of these rods and

couples, is that they fit nicely and can be set up so as not to wobble. By using several rods and

couplers a universal motion can be secured, but not so conveniently, as by the ball-joint clamps

and swivel couplers made for photographers' use by Otis C. White, of Worcester Mass. These

allow extreme freedom or movement, and when fastened do not slip nor wobble. The ball joints

are made to clamp on the edge of the table or to screw upon the end of rods. The first can be

had in great variety of sizes, a convenient one fitting half inch rode costing $1.25. The swivel

couplers allow the coupling of the rode in any position relative to each other, those of size to

connect half inch and quarter inch rods costing 50 cents. Rods of various diameter and length

may also be had with the ball-joints and swivel clamps. In purchasing for a laboratory from

several makers it would be well to fix upon standard sizes for rods and fittings so that all may

be interchangeable; and also to fix upon a standard size and number of threads to the inch for

all screws cut upon the rods so that any clamps, pulleys or other small pieces of apparatus,

made to screw upon one, [p. 477]

On Vision in general cf. Helmholtz, Handbuch der physiologischen Optik. (The second German

edition has reached page 400: the latest complete edition is the French translation. Optique

Physiologique, Paris. 1867). Aubert. Grundzüge der physiologischen Optik, Leipzig, 1876. (a

portion of Graefe and Saemisch's Handhuch der ges. Augenheilkunde). Le Conte, Sight, New

York. 1881. Beaunis, Nouveaux Éléments de Physiologie Humaine. Paris. 1888, (Beaunis, like

Helmholtz, gives bibliographies). Wundt. Physiologische Psychologie. II, 82-209. Hermann's

Handbnch der Physiologie. Bd. III, Th. 1, Leipzig. 1879.

The references following the experiments below are made chiefly to Helmholtz, the pages of

the new German edition, the French edition, and, in parenthesis following the latter, of the first

German edition being given, but the experiments of this section are more or less fully discussed

In almost all of the works just mentioned and in many others besides.

99. The retinal image. The mechanisms of the eye accomplish two things: the projection of a

well defined image on the retina; and the ready shifting of the eye so as to bring successive

portions of the image into the best position for vision. The retinal image is readily seen in the

unpigmented eye of a pink eyed rabbit. Chloroform the rabbit, remove the eyes and mount

them in clay for readier handling. Make a thick ring of clay with an internal diameter a little

greater than that of the comes of the rabbit's eye, place the eye comes downward in the ring

and lay a similar ring upon it to keep it in place. It can now be handled easily and turned in any

direction. Turn it toward the window and from behind observe the inverted image on the retina.

Bring the hand into range and move it to and fro observe that the image of distant objects is

more distinct than that of the hand. If convex and concave lenses are at hand (spectacle lenses

will answer) bring them before the eye and observe that the effect upon the retinal image is

similar to that seen subjectively when they are held before the observer's own eye. Reverse the

eye, holding it retina side toward the window, and observe the radiating and circular fibres of

the iris. The eye must be fresh, for if long removed it loses its transparency.

100. Accommodation. The sharpness of the retinal image depends on the adjustment or the

crystaline lens, which must be such as to focus the light from the object under regard upon the

retina. The lens must be thicker and rounder for near objects, thinner and flatter for more

distant ones. These adaptations of the eye are known as Accommodation. The changes in the

clearness of the retinal image are easy to observe subjectively. Hold up a pin or other small

object six or eight inches away from the eyes. Close one eye and look at the pin with the other.

The outline of the pin is sharp, but the outlines of things on the other side of the room behind it

are blurred. Look at these and the outline of the pin becomes blurred. Notice the feeling or

greater strain when looking at the nearer object. The experiment is somewhat more striking

when the nearer object is a piece of veiling or wire gauze and the farther a printed page.

On this and the next two experiments cf. Helmholtz, Physiologische Optik, 2nd Ed. pp. 112-118,

French ed. pp. 119 (90)-126 (96).

101. Accommodation. Scheiner's experiment. a. Pierce a card with two fine holes separated by

a less distance than the diameter of the pupil, say, a sixteenth of an inch. Set up two pine in

corks distant respectively eight and twenty inches in the line of sight; close one eye and holding

the card close before the other with the holes in the same horizontal line look at the nearer pin;

the farther pin will appear double; look again at the nearer pin and while looking cover one of

the holes with another card; one of the images of the farther pin will disappear, the left when

the left hole is covered, and the right when the right is covered. Look at the farther pin or

beyond it and repeat the covering, covering the left hole now destroys the right image of the

nearer pin and covering the right destroys the left. Why this should be so will be clear from the

diagrams below. The upper diagram illustrates [p. 478] the course of the rays of light when the

eye is accommodated for the nearer pin; the lower diagram when it is accommodated for the

farther pin. A and B represent the pins; S and S the pierced screen; d and d' the holes in the

screen; c and c the lens; a'ba" and b"ab' the retina; A', A", B', and B", the positions of the

double images; the solid lines the course of the rays from the pin accommodated for; the dotted

lines the course of the rays from the other pin; the lines of dashes the lines of direction, i.e.,

those giving the direction in which the images appear to the observer. In the upper diagram the

rays from B are focused to a single retinal image at b, while those from A, being less divergent

at first, are brought to a focus nearer the lens, cross over and meet the retina at a' and a", and

since each hole in the screen suffices to produce a retinal image, cause the pin to appear

double, and its two images are referred outward as usual with retinal images along the lines of

direction, (which cross a little forward of the back surface of the lens, in the crossing point of

the lines of direction), the right retinal image corresponding with the left of the double images

and vice versa. If now the right hole in the screen be closed the left retinal image and the right

double image disappear. The case of accommodation for the farther pin will be clear from the

lower diagram, if attention is given to the dotted and dashed lines. It will also be easy to explain

why moving the card when looking through a single pin hole causes apparent movements or

the pin not accommodated for, and why in one case the movement seems to be with the card

and in the other case against it. b. Stick the pins into the corks so that they shall extend

horizontally, and examine them with the card so held as to bring the holes above one another.

c. Arrange the holes thus: and observe that the triple image of the nearer pin (when the

farther is fixated) has the reverse figure Scheiner's experiment can easily be illustrated

with a double convex lens and a pierced screen of suitable size.

102. The Range of accommodation. a. Find by trial the nearest point at which a pin seen, as In

Ex. 101, can be seen single. This is the near point of accommodation. For the short-sighted a

far point may also be found, beyond which double images reappear. b. Find how far apart in the

line of sight two pins may be and yet both be seen single at one and the same time. Try with

the nearer at 20 cm., at 50 cm., at 2 m. That portion of the line of sight, for points in which the

same degree of [p. 479] accommodation is sufficient, is called the line of accommodation. The

length of the line Increases rapidly as the distance of the nearer object from the eye increases.

Cf. Helmholtz, op. cit. G. 114, 119. Fr. 122 (93), 123 (97).

103. The Mechanism of accommodation. a. The change in the lens in accommodation is chiefly

a bulging forward of its anterior surface. This may be observed as follows. Let the subject

choose a far and a near point of fixation in exactly the same line or vision, class one eye and fix

the other upon the tar point. Let the observer place himself so that he sees the eye or the

subject in profile with about half the pupil showing. Let the subject change his fixation at

request, from the far to the near point, being careful to avoid any sidewise motion of the eye.

The observer will then notice that more of the pupil shows and that the farther side of the his

seems narrower. This change

is due to the bulging forward

of the front of the lane. If the

change were due to

accidental turning of the eye

toward the observer the

farther edge or the iris should

appear wider instead of

narrower. b. Purkinje-Sanson

images. The changes in the

curvature of the lens may also

be observed by means of the

images reflected from its front

or back surfaces and from the

front of the cornea. Operate in

a darkened room or at night.

Let the subject choose far and

near fixation points as before.

Let the observer bring a

candle near the eye of the subject at a level with it and a little to one side and place his own

eye in a position symmetrical to the candle on the other side of the subject's line of eight.

Careful examination will show three reflected images of the flame; one on the side of the pupil

next the light, easily recognizable, bright and erect, reflected from the surface of the cornea; a

second nearer the centre of the pupil and apparently the farthest back of the three, erect like

the first, but very indistinct, (more like a light cloud than an image), reflected from the anterior

surface of the lane; and a third, a mere point of light, near the side of the pupil farthest from the

flame, inverted and reflected from the posterior surface of the lens. When the observer has

found these three images the subject should fixate alternately the near and far points chosen.

As he fixates the near point the middle image will grow smaller, advance and draw toward the

corneal image; when he fixates the far point the image will enlarge, recede and move away

from the corneal image. The following diagram after Aubert illustrates the movement of the

middle image; the full lines indicate the positions of the comes and lens and the course or the

rays of light when the eye is accommodated for the far point; the dotted lines indicate the

anterior surface of the lens and the direction or the ray reflected from Its surface when the eye

is accommodated for the near point. Three images similar to those in question can be observed

on a watch glass and a double convex lens herd In the relation of the comes and crystaline.

Cf. Helmholtz, op. cit. G. 131-141. especially 131-134. Fr. 142 (104)-154 (112), especially 142

(104)-146 (101). Aubert, Physiologische Optik. 444.

104. Chromatic aberration. Of the various defects of the eye as an optical instrument only one

will be mentioned here, namely, chromatic aberration, and that because it has been supposed

to offer a possible means of inferring the relative distance of objects from the eye. The [p. 480]

different colored rays of light are not equally retracted by the lens, the violet most, the red least,

and the other colors In order between. The point at which parallel violet rays are brought to a

focus is therefore nearer the lens than the point for red; and in order that the same degree of

accommodation may serve to show a red lighted object and a violet lighted object at the same

time and both with full distinctness, the red must be somewhat farther away. a. The aberration

can easily be observed by looking at a small gas or candle flame through a piece of cobalt blue

glass which transmits light from the two ends of the spectrum chiefly. Hold the glass eight or

ten inches before the face and fixate some point on it the flame will appear pinkish with a blue

border. Fixate some point considerably beyond the flame; the flame is now bluish and the

border is a fine red line. b. Look at the edge of the window frame next the pane, and bring a

card before the eye so about half the pupil is covered; if the card has been brought up from the

frame aide, the frame will be bordered with yellow; if from the pane side, with blue. In ordinary

vision these fringes do not appear, because the colors overlap one another and produce a

practically colorfess [sic] mixture. c.v. Bezold's experiment. Something similar may be

observed, on regarding the parallel lines of the left figure under Ex. 111 with imperfect

accommodation.

Cf. Helmholtz. op. cit. G. 156-164; Fr. 172 (125)-119 (131). Beaunis, Nouveaux éléments de

physiologie humaine. II. 506. v. Bezold, v. Graefe's Archiv F. Ophthalm., XIV, Heft 2, 1-29.

105. Accompanyments or accommodation. a. Notice that as the subject in Ex. 103

accomodates [sic] for a near point, his pupil grows smaller, and as he accommodates for a far

point, grows larger. Cf. Also Ex. l06, b. Degrees of accommodation suitable for objects at

different distances are habitually associated with the amounts of convergence of the lines of

sight necessary to fix the eyes upon such objects, and a little practice is necessary before the

convergence and accommodation can be dissociated. Place a couple of postage stamps six

inches apart on the table and look at them from a distance of twelve or fifteen inches with

crossed eyes so that the left eye looks at the right stamp and the right eye at the left stamp; the

lines of sight now cross only a few inches from the eyes and the accommodation is for that

distance and not for the true distance of the stamps, as is betrayed by the blurring of their

images. Holding a pencil at the crossing point of the lines of sight is helpful in first attempts at

crossed vision.

Cf. Helmholtz, op. cit. G. 130. Fr. 142 (104).

106. Entoptic phenomena: Muscae volitantes, etc. Fix a lens of short focus at some distance

from a bright gas or candle flame. a. Set up in the focus of the lens a card pierced with a very

fine hole, bring the eye close to the hole and look toward the light; the eye should be far

enough from the hole to prevent the edge of the lens from being seen; the rays of light that now

reach the eye are divergent and the crystaline lens does not bring them to a focus on the retina,

but only refracts them to such a degree that they traverse the eye nearly parallel and thus in

suitable condition for casting sharp shadows upon the retina or objects on or in the eye. The

lens will appear full of light, and in it will be seen a variety of shadings, blotches and specks,

single or in strings, the outward projection of the shadows just mentioned. The figures in this

luminous field will vary from person to person, even from eye to eye, but in almost every eye

some will be found that move and some that remain fixed and only move with the eye. Of the

moving figures some are due to particles and viscous fluids on the surface of the eye; they

seem to move downward and are changed by winking. Notice for example the horizontal bands

that follow a slow dropping and raising of the upper [p. 481] lid. Others, the muscae volitantes

are frequently noticed without any apparatus, they appear as bright irregular threads, strings of

beads, or groups or points, or single minute circles with light centres. They seem to move

downward in the field and consequently actually move upward in the vitrious humor where they

are found. Of the permanent ones, some are due to irregularities of structure or small bodies in

the lens and its capsule (spots with dark or bright centres, bright irregular lines, or dark

radiating lines corresponding probably to the radial structure of the lens); others of a relatively

permanent character can be produced on the cornea by continued rubbing or pressure on the

eyeball. b. The round spot of light in which these things are seen represents the pupil, and the

dark ground around it the shadow or the iris. Notice the change in the size of the spot of light,

as the eye is accommodated for different distances (cf. Ex. 105), and as the other eye is

exposed to, or covered from, the light. The change begins in about halt a second. It shows the

close connection of the iris mechanisms or the two eyes and is typical of the way in which the

two eyes co-operate as parts of a single visual machine. Some of these entoptic observations

may be made with a pierced card alone, or simply by looking directly at a broad expanse of

clear sky with out any apparatus at all.

Cf. Helmholtz, op. cit. G. 184-192 and Tafel I. which represents the appearance of several of

the entoptic objects; Fr. 204 (149)-214 (156) and Pl. V; also pp. 548 (419)-558 (427).

107. Retinal blood-vessels, Purkinje's vessel figures. a. Concentrate a strong light, (preferably

in a dark room) or even direct sunlight, with a double convex lens of short focus on the sclerotic

in the outer corner of the eye of the subject, requesting him to turn the eye toward the nose and

giving him a dark background to look toward. Make the spot of light on the sclerotic as small

and sharp as possible and give to the lens a gentle to and fro or circular motion, and after a

little the subject cannot fail to see upon the field which the light makes reddish yellow the dark

branching figure of the shadows of the retinal vessels. Notice that the area directly fixated, is

partially surrounded, but not crossed by the vessels. In this lies the yellow spot (macula lutea)

or area of clearest vision of the retina, not, however, to be observed in this experiment. The

centre from which the vessels radiate lies in the point or entrance of the optic nerve. In this form

of the experiment the light radiates in all directions within the eye from the illuminated point of

the sclerotic. b. Somewhat the same kind of an image of the vessels is to be secured by

moving a candle about near the eye, below it and a little to one side. In this experiment some

indication of the region of the yellow spot is to be seen. In this form of the experiment the light

enters by the pupil, forms an image on a part of the retina somewhat remote from the centre

and this retinal image is the source of light by which the vessel shadows are cast. c. Look

through a pin hole in a card directly at the clear sky or any other strongly illuminated even

surface or at a broad gas flame. Give the card a rather rapid circular motion and the finer retinal

vessels in the region of the yellow spot will readily be seen, among them also a small colored or

slightly tinted spot (best seen perhaps by gas light) representing the macula, and in its centre a

shadowy dot (representing the fovea or point of clearest vision) which appears to rotate when

the motion of the card is circular. If the card is moved horizontally the vertical vessels alone

appear; it vertically, the horizontal vessels. Notice also the granular appearance of the macula;

the granulations have been supposed to represent the visual cones of that region. The finer

retinal vessels can also be seen when looking at the vacant field or a compound microscope, it

the eye is moved about rapidly. In all of these cases it is important that the shadows be kept

moving; it they stand still, they are lost. The explanation [p. 482] is partly physiological, the

portions of the retina on which the shadows rest soon gain in sensitiveness enough to

compensate for the less light received, and partly psychological, moving objects in general

being more readily attended to, and those whose images rest continously [sic] on the retina

without motion being particularly subject to neglect. Once having become acquainted with the

appearance of these vessel figures it is often possible to see traces of them without any

apparatus. Parts of them, with something of the projection of the yellow spot, map sometimes

be seen for an instant as dark figures on the diffusely lighted wells and ceiling or as light figures

on the dark field or the closed eyes when the eyes are opened and closed after a glance at the

window on first waking in the morning, or in blue when looking at the snow and winking on a

bright morning, or projected on the sky and keeping time with the pulse after a rapid walk up

hill.

Helmholtz op. cit. G. 192-198. Fr. 214 (156)-211 (161).

108. Retinal circulation. Look steadily through two or three thicknesses of blue glass at the

clear sky or a bright cloud, and observe a large number of what seem to be bright points darting

hither and thither like bees in a swarm or rapidly blown snow-flakes. Careful observation will

also establish that the bright points are followed by darker shadowy ones. Pick outs speck on

the window to serve as a fixation point, look at it steadily and observe that while the movements

or the points seem irregular the same lines are retraced by them from time to time. When

several of their courses have been accurately observed, repeat the experiment for

demonstrating the finer retinal vessels (Ex. 107 c.) and notice that fine vessels are found which

correspond to the courses which the points seem to follow. These flying points can be seen

without the glass by a steady gaze at an evenly lighted bright surface, and some times a

rhythmic acceleration of their movement will be found, corresponding to the pulse. Helmholtz

explains the phenomenon as due to the temporary clogging of fine capillary vessels by large

blood corpuscles. The bright lines (the apparent tracks of bright points) are really the relatively

empty capillary tubes ahead or the corpuscles, which, after an instant, are driven onward by

others crowding behind and in turn give the shadow that apparently follows the bright points.

Cf. Helmholtz, op. cit. G.198; Fr. 221 (837), 555(425), Rood, American Journal of Science, 2d

Series, XXX, 1860, 264-265, 385·386.

109. The Blind-spot. Mariotte's experiment. The point of entrance or the optic nerve is

unprovided with visual end-organs and is irresponsive to light. a. This insensitiveness is easily

demonstrated with the diagrams below. Close the left eye and keeping the right fixed on the

asterisk in the upper diagram move it backward and forward till a point is found where the black

oval disappears. For the blind spot of the [p. 483] left eye use the second diagram. The blind

spot may be demonstrated simultaneously in both eyes by the use of a figure like that below

enlarged a couple of times. The experimenter should look at the asterisk while he holds a sheet

of paper in the median plane of his head, to prevent each eye from seeing the other's part of

the diagram.

b. To draw the projection of the blind-spot, arrange the head support described above, piece

opposite the face at a distance of about 18-inches, a vertical sheet or white paper and put a dot

on it for a fixation point. Fasten upon the end of a light rod a bit of black paper about 2 mm.

square or blacken the end of the rod with ink. Bring the face into position, close one eye, and fix

the other upon the dot. Move the rod slowly so as to bring the little square over the part or the

white paper corresponding to the blind spot, dotting on the paper the points where the square

disappears or reappears. Repeat at various points till the outline of the projection of he blind

spot is complete. If the mapping is carefully carried out, the map will probably show the points

of departure of the large blood vessel, that enter with the nerve.

Helmholtz, op. cit. G. 250-254, Fr. 284 (210)-288 (214).

110. The yellow spot, macula lutea. The projection of the yellow spot in the visual field can be

made visible in several ways. Two have already been mentioned in Ex. 107; others are as

follows. a. Close the eyes for a few seconds and then look with one or them through a flat sided

bottle of chrome alum solution at a brightly lighted surface (not yellow) or the clear sky. In the

blue green solution a rose colored spot will be seen which corresponds to the yellow spot. The

light that comes through the chrome alum solution is chiefly a mixture of red and green and

blue. The pigment or the yellow spot absorbs a portion of the blue and green and transmits the

rest, which makes a rose colored mixture, to the visual organs behind it. b. The region of the

yellow spot may be seen as an area of somewhat deeper shade when the eye looks at on

evenly lighted surface like the ceiling, and the illumination is made intermittent by moving the

spread fingers to and fro between the eye and the ceiling.

Cf. Helmholtz, op. cit. Fr. 548 (419)-551 (421). On a. cf. Maxwell, On Color-vision at different

points of the Retina, Report of the British Assoc., 1870; or Vol. II, pp. 230-232 of Maxwell's

Scientific Papers. Cambridge, 1890.

111. Visual cones in the fovea. Bergmann's experiment. Place the left hand diagram in a good

light and look at it from a distance of a yard and a half or two yards. Observe the apparent

bending and beading of the lines. This is supposed to be due to the mosaic arrangement [p.

484] or the visual cones. The cones that are touched by the image of one of the white lines are

stimulated in proportion se are more or less touched. Those that are much stimulated furnish

the sensation of the white line and its irregularities, those that are little stimulated join with

those that are not touched at all to give the image or the black line and its irregularities. This is

schematically represented in the right hand cut.

Cf. Helmholtz, op. cit., G. 257-258, Fr. 293 (217)-294 (218). Bergmann, Zeitschrift für rat. Med.,

(3), II, 88.

112. Acuteness of vision, minimum visibile, and size of the cones in the fovea. Place the

parallel line diagram used in the last experiment in a good light and walk backward from it till

the lines can just no longer be distinguished as separate. If the experimenter's eyes are not

normal he should use glasses that fit his eyes for distinct vision at the distance required.

Measure the distance between the eye and the diagram and calculate the angle whose apex

lies in the crossing point of the lines of direction (about 7 mm. back of the cornea and 16 mm. in

front of the retina) and whose base is the distance from the middle of one line of the diagram to

the middle of the next; in this diagram 1.58 mm. This angle measures the least visible extent

when discrimination is involved; the least luminous extent that can still impress the retina is far

smaller, as witness the visibility or the stars. On the supposition that if the sensations of two

cones are to be separable they must be separated by an unstimulated, or at least by a less

stimulated, cone, it has generally been considered that the cones could not subtend a greater

angle than that found in this experiment, 60"-90" representing 0.004-0.006 mm. on the retina,

and this agrees well with microscopical measurements. But as Helmholtz notices(Phys. Opt.

2nd ed. p. 260) this experiment does no more than prove that there are on the retina rows of

sensitive elements the middle lines of which are separated by the angular distance found in the

experiment. The elements themselves, if properly arranged may be somewhat larger.

Calculation of the number of such elements in a sq. mm. of the retina based on this view of the

experiment agrees well in the case of Helmholtz's own determination with the result of

microscopical counting. b. The discriminative power or the retina falls off rapidly in all directions

from the fovea, more rapidly above and below than in a horizontal direction. Arrange a head

rest and perpendicular plane as in Ex. 109 b. Place upon the end of the rod used in that

experiment a card on which have been made two black dots 2 mm. in diameter and 4 mm. from

centre to centre. Move the card horizontally toward the fixation point, beginning beyond the

point at which the two dots can be distinguished and moving inward till they can just be

distinguished. Measure the distance from the fixation point and repeat several times both to the

right and left of the fixation point and above and below, holding the card so that both dots are in

each case equally distant from the fixation point.

Helmholtz, op. cit., G. 255-264, Fr. 291 (215)-301 (223).

113. Mechanical stimulation of the retina. a. Phosphenes. Turn the open or closed eye as far as

possible toward the nose and press on the eyelid at the outer corner with the finger or the tip of

a pen holder. On the opposite side of the visual held will be seen a more or less complete circle

of light surrounded by a narrow dark band, outside of which again is a narrow band of light.

Notice the color of the light seen. Get the phosphenes by pressure at other points of the eye

ball. b. Press the eye moderately with some large object, say the angle of the wrist when the

hand is bent backward, and continue the pressure for a minute or two. Peculiar palpitating

figures will be observed and [p. 485] strange color effects. The former Helmholtz compares to

the tingling of a member that is "asleep." c. Standing before a window close the eyes and turn

them sharply from side to side. As they reach the extreme position in either direction observe

immediately In front of the face a sudden blue spot surrounded by a yellow band. A second

fainter spot farther from the centre in the direction of motion may also be seen. The yellow ring

is due to the stimulation of the portion of the retina in the region of the blind spot in the eye that

turns inward. The blue spot represent the blind spot in the same eye. Cf. Explanation in the

latter part of Ex. 115.

Helmholtz, op. cit. G. 235-259. Fr. 266 (196)-270 (200). Le Conte, American Journal of

Psychology, III. 1889-90, 364-366.

114. Idio-retinal light, light chaos, light duet. Close and cover the eyes so as to exclude all light,

or experiment in a perfectly dark room. Let the after effects of objective light fade away and

then watch the shifting light clouds of retinal light. The cause of the retinal light is not altogether

clear, but it is supposed to be a chemical action of the blood on the nervous portion of the

visual apparatus. Aubert estimates its brightness at about half the brightness of a sheet of

paper illuminated by the planet Venus when at its brightest. b. When awake in the night time in

a room that is almost perfectly dark (e.g. in which the form of the window and the large pieces

of furniture cannot be made out) notice that the white clothing of the arms can be seen faintly

as they are moved about, but not when they are still. In the last case the very faint light they

reflect is not sufficient to make them distinguishable from clouds of idio-retinal light.

Cf. Helmholtz op. cit. G. 242-243, Fr. 274 (202)-275 (203). On b. cf. Helmholtz. Die Störung der

Wahrnehmung kleinster Helligkeitsunterschiede, Zeitschrift für Psyhologie. I, 1890, 6-9.

115. Electrical stimulation of the visual apparatus. Moisten thoroughly with strong salt water

both the electrodes and the portions of the skin to which they are to be applied. Place one of

the electrodes on the forehead (or on the edge of the table and lay the forehead upon it), the

other on the back of the neck; or, if the current is strong enough, hold it in the hand or lay it on

the table and put the hand upon it. At each opening or closing of the circuit a bright flash will be

seen, whether the eyes are closed or open. With the eyes closed and covered the effects of the

continuous current may be observed. In this case it is well to apply the electrode slowly and

carefully so as to avoid as much as possible the flash caused by the sudden closing of the

circuit. When the positive electrode is on the forehead, the negative on the back of the neck a

transient pale violet light will be seen distributed generally over the field and forming a smell

bright spot at its centre. Sometimes traces of the blind spot appear. The violet light soon fades

and on opening the circuit, there is a notable darkening of the held with a momentary view of

the blind spots as bright disks. When the negative electrode is on the forehead, the: positive on

the back of the neck, the phenomena are in general reversed, the darkening occuring [sic] on

closing the circuit, the violet light on opening it. Helmholtz sums up these and other

experiments as follows: "Constant electrical circulation through the retina from the cones

toward the ganglion cells gives the sensation of darkness, circulation in the contrary direction

gives the sensation of brightness." (Phys. Opt. 2nd ed. 247). That the blind spot should appear

as a disk of different color from the rest of the held seems to be due to the fact that the

sensitive parts of the retina immediately surrounding it are somewhat shielded from the electric

current, and as usual their condition is attributed to the blind spot also. The experiment is not

entirely a pleasant one, on account of the feeling [p. 486] which the current produces in the

head, the electrical taste in the month and the reddening of the skin under the electrodes.

Cf. Helmholtz, op.·cit. G. 243-248, Fr. 275 (203)-281 (207).

116. After-images, accidental or consecutive images, After-images in which the relations of light

and shade of the original object are preserved are called Positive After-images. Those in which

these relations are reversed (as in a photographic negative) are called Negative After-images.

Positive after-images are of changing colors, but most important to notice here are those of the

color of the object (like colored), and of the complementary color (opposite colored). Negative

after-images, so far as observed, are always opposite colored. All after-images, especially the

positive, can best be observed in the morning when the eyes are well rested. a. Negative after-

images: look steadily for a minute at a fixed point or the window, then at a white screen or an

evenly lighted unfigured wall; the dark parts of the window will now appear light and vice versa.

Get a lasting after-image and look at a corner of the room or at a chair or other object of

uneven surface; notice how the image seems to at itself to the surface upon which it rests. After

a little practice it is also possible at desire to see the image floating in the air instead of lying on

the background. b. Look steadily at a bright colored object or some bits of colored paper, then

at the screen; observe that the colors of the after-images are approximately complementary to

the colors of the objects producing them. Negative after-images are some times very lasting

and for that reason are those most frequently noticed in ordinary experience; they are a

phenomenon of retinal fatigue. c. Positive after-images. Look for an instant (one-third of a

second) at the window, then close and cover the eyes, or look at a dark surface; for a very

short time an after-image like the original object in color and distribution of light and shade can

be seen. The positive after-image is of short duration and is not so readily observed as the

negative; it is a phenomenon of retinal inertia, or the prolongation of retinal excitation. d.

Colored positive after-images. Look for an instant at a gas flame through a piece or red glass,

then close the eyes and observe the red image; repeat the experiment continuing the fixation of

the flame for half a minute; the resulting after-image will be bright as before but of the

complementary color. e. Get an after-image of the window of not too great an intensity, and

alternately project it on a sheet of white paper and the dark field of the closed and covered

eyes; it will be found negative on the white back-ground and positive on the dark. f. Get a good

after-image of the window and observe with closed and covered eyes the play of colors as the

image fades. Try several times and observe that the order of succession is the same.

Cf. Helmholtz, op. cit. Fr. 446 (338), 471 (357)-500 (380). Wundt, Physiologische Psychologie,

3rd ed I,472-476.

117. Effect of eye-motions on after-images. Get a moderately strong after-image of the window;

look at the wall and keep the eyes actively in motion; the image will be seen with difficulty while

the eye is in motion; when the eye is brought to rest, however, it will soon appear. In general

any visual stimulus that moves with the eye is less effective than one that does not.

Cf. Exner, Das Verschwinden der Nachbilder bei Augenbewegungen. Zeitschrift für

Psychologie, I, 1890, 47-51.

118. The seat of the after-image. An after-image due to exclusive stimulation of a single eye

may under proper conditions sometimes seem to be men with the other unstimulated eye. From

this it has been inferred that the seat or after-images was central, not peripheral; that is, in the

visual centres of the brain, not in the [p. 487] eye. The following experiments show, however,

that the after-image is really seen with the eye first stimulated, and so render the hypothesis of

a central location unnecessary. a. Look steadily for several seconds at a bit of red paper on a

white ground, using only one eye, say the right, and keeping the other closed; when a strong

after-image has been secured, remove the paper, close the right eye, open the left and again

look steadily at the white ground; after a little the field will darken and the after-image will

reappear. If the red does not produce a sufficiently lasting image, substitute for it a gas flame or

some other bright object. That we have really to do, however, with the eye originally stimulated,

(its present dark held being superposed upon the light one of the other eye) appears from the

results of b and c. Get the after-image as before; then open both eyes and bring a bit of card-

board before the eyes alternately; bringing it before the left eye rather brightens the image

bringing it before the right dims or abolishes it; the image is therefore chiefly affected by what

affects the right eye. c. Get the after-image again and close and cover both eyes; observe the

color of the after-image as projected on the dark held; then open the left eye, letting the right

eye remain closed and covered; the after-image will be seen, not in the color it has when the

right eye is open and the image is projected in the light field, but in that which it has in the dark

field of the closed eye.

Cf. Delabarre. On the seat of Optical After-Images, American Journal of Psychology, II. 1888-

89, 326-328.

119. After-images of motion. Fasten upon the rotation apparatus a disk like that in the first cut

on page 475. Then look at a page of print or into the face of a by-stander and notice the

apparent shrinking (if the spiral has seemed to run outward) or swelling (if the spiral has

seemed to run inward). Illusions of increase or decrease of distance sometimes accompany

those of motion. These after-images of motion have been explained as due to unconscious

persisting movements of the eyes. This is probably incorrect, for in the present case it would

seem necessary that the eyes should move in all directions at the same time.[2]

Cf. Helmholtz, op. cit. Fr. 766 (603)-769(605). Bowditch and Hall, Optical Illusions of Motion,

Journal of Psychology, III, 297-307. Mach, Bewegungsempfindungen, Leipzig, 1875, pp. 59-61.

(see also pp. 61-65 for yet another kind of after-image) and Analyse der Empfindungen, Jena,

1886, pp. 65-67.

120.

Irradiation.

This term is

used to

designate the

apparent

enlargement of

bright surfaces

at the expense

or adjacent

dark surfaces.

It is most

strongly

marked when

the bright

surface is

intense and the accommodation is imperfect, but is not absent with perfect accommodation.

Even with perfect accommodation, and much more so with imperfect accommodation, the line

of juncture of a bright and dark surface is not really a sharp line but a narrow band of gray of

which more than [p. 488] the proper amount is credited to the white, for reasons to be brought

out in the section to follow on the Psychophysic Law. The following are some of the common

cases of irradiation: a. Hold a ruler or a straight edged piece or black card-board close before a

gas or candle flame so se to cover a portion of it, and notice that the flame seems to cut into

the edge, and if there are differences in brightness the brightest parts cut in deepest. b. Notice

that the white squares in the diagram below, when brought into a strong light, seem larger than

the black, though they measure the same in size.

c. Irradiation of dark lines. A black line on a white surface (or a white line on a black surface)

may some times be enlarged by the greater part or its gray fringe, because near the outer edge

of the fringe the blackness (or for white lines, the whiteness) decreases very rapidly and so

seems to make a boundary. Look at the accompanying diagram through a lens that will make

accommodation very imperfect. The narrow black strips will appear larger for the reason just

mentioned, while the lower black areas will be cut into as in the ordinary cases of irradiation,

giving to the white stripe between the shape of a club with the handle uppermost. Helmholtz

suggests with reason that these two phenomena, having quite different causes, should have

different names, and the term "irradiation" be confined strictly to such enlargement of white

surfaces as takes place with exact

accommodation.

Cf. Helmholtz, op. cit., G. 394-400, Fr. 425

(321)-433 (327).

121. Reflex movement of the eye. The eye is

a mooing as well as a seeing member and its

motor functions are of great importance for

psychology. Of the first importance is the

constant reflex tendency of the eye to move

In such a way as to bring any bright image

lying on a peripheral part of the retina, or any

to which attention is directed, into the area of

dearest vision. Many evidences of this

tendency will be found in the ordinary course

of vision. By way of experiment, try to study

attentively a musca volitans or a negative

after-image that is just to one side of the

direct line of sight. The apparent motion of

the object measures the energy of the reflex.

122. Associated movements of the eyes. The two eyes form a single visual instrument and

even when one eye is closed it follows to a considerable degree the movements of its open

companion. a. Close one eye and, resting the finger-tip lightly on the lid, feel the motions of the

eye as the other looks from point to point of the visual held. b. Get a monocular after-image as

in Ex. 118 and when it has become apparently visible to the open eye, notice that it seems to

accompany that eye as it takes one fixation point after another in the field of regard.

123. Motions of the eyes when the lines of sight are parallel, Donders's and Listing's laws. All

motions of the eye can be interpreted as rotations of greater or less extent about one or more

of three axes: a sagittal axis, corresponding nearly with the line of sight; a frontal axis,

extending horizontally from right to left; and a vertical axis. All these intersect in the centre of

rotation of the eye. Now it is easily conceivable that for any position of the line of sight, e.g. 15°

to the right and 10° upward, there would be an infinite number of positions that the eye might

assume by rotation about the line of sight itself. As a matter of fact, however, it does not

assume an indefinite number of positions, but one and only one, no matter by what route the

line of sight may have come to that point. This is the law of constant orientation [p. 489] or

Donders's law. Listing's law goes further and asserts that the position is not only fixed, but is

such as the eye would assume if the line of sight were moved from its primary position

(approximately that in which the eye looks straight forward to the horizon) to the point in

question without any rotation at all about the line of eight, but about a fixed axis standing

perpendicular at the centre of rotation to both the primary and the new position of the line of

sight. The advantage to vision of the constancy or orientation and the exclusion of rotation

about the line of sight is considerable, especially in determining directions in the held of regard.

The correctness of these laws is easy to demonstrate, a. Donders's law. Cut in a sheet of black

cardboard two slits an eighth of an inch wide and six or eight inches long, crossing at right

angles. Set the cardboard in the window or before some other brightly lighted surface. Arrange

a head rest at some distance and when the head is in position, get a strong after-image of the

cross, fixating its middle point. Then, without moving the head, turn the eyes to different parts of

the walls and ceiling. The image will suffer various distortions from the different surfaces upon

which it is projected, but each time the eye returns to the same point the image will lie as

before. If the wall does not offer figures by which this can be shown, have an assistant mark the

position of the image upon it. The after-image is of course fixed on the retina and can move

only as the eye moves. b. Listing's law. Make over the cross used in a into an eight rayed star

by cutting two other narrow slits across its centre. Arrange the card before a brightly lighted wall

and parallel to it at a height a little less than that of the eyes when the head is in position. Draw

lines or stretch threads on the wall that shall appear to continue the rays of the star upward and

right and left, and downward if convenient. Fix the head rest directly before the star at a

distance of five or six yards or more, adjust the head so that when the after-image of the star is

carried along the horizontal or vertical line its corresponding ray will coincide exactly with the

line. When this condition is fulfilled for both lines the eyes and lines of sight are in the primary

position. When the primary position has been found, carry the after-image along the lines

prolonging the other rays and observe that as before the after-image of the ray coincides with

its line. This would be found true, for all except extreme positions, of all other rays, and shows

that the eye does not in such motions rotate about the line of sight. c. In motions from other or

secondary positions, however, there Is such a rotation. Turn the head somewhat to one side or

tip it forward or backward from the primary position repeat b and notice that the lines of the

after-image betray some rotation.

Cf. Helmholtz, op. cit., Fr. 601 (462)-610 (470), 621 (479) ff. Le Conte, Sight, pp. 164-177.

124. Actual movements of the eyes. Rapid

motions of the eyes are not executed with

mechanical exactness according to Listing's law,

though it gives correctly the end position

reached. The axis of rotation is not quite

constant and the lines passed over by the point

of sight are therefore not quite straight. This is

easy to observe; as follows. In a dark room turn

down the gas till it burns in a flame not more than 8 or 10 mm. high. Then using this as a point of

departure in the primary position look suddenly from it to other points of fixation in various

directions about it, and notice the shape of the long positive after-images that result from the

motion of the image of the flame, over the retina. These will probably have the shape of the radii in

the left hand figure below. The newest part of the after-image is that next the light, the oldest part

is that next the fixation point, for example at a. If the points of the after-image curve are now

interpreted in the order or time, it [p. 490] appears that the eye at first moved rather rapidly toward

the right but rather slowly upward, while at last it moved rather slowly toward the right and rapidly

upward. Plotting the curve accordingly we get the reverse curve shown in B which shows the true

track of the fixation point. It is said that for some eyes the after-images, though curved, do not

coincide with those figured in A.

Cf. Wundt. Beiträge zur Theorie der Sinneswahrnehmung, Leipsig[sic], 1862, pp. 139 ff. 202.

Hermann's Handbuch der Physiol. III, Th. 1. 450-451.

125. Convergent movements of the eyes. When the lines of sight converge, the movements of

the eye do not follow Listing's law. When the lines of sight converge in the primary position both

eyes rotate outward; as the lines of sight are elevated, the convergence remaining the same,

the outward rotation increases; as they are depressed, the rotation diminishes and finally

becomes zero. On a sheet of cardboard draw a series of equi-distant parallel vertical lines one

or two inches apart and eight or ten inches long, drawing the left half of the group in black ink,

the right half in red. Cross both sets midway from top to bottom by a horizontal line, red in the

red set and black in the black set. Fasten the cardboard fist upon a vertical support and arrange

the head rest in front of it. The horizontal line of the diagram should be on a level with the eyes.

a. Fasten a bit of wire vertically between the eyes and the diagram in such a way that it can be

moved to and from the eyes. Bring the head into position and look at the wire, but give attention

to the diagram. It will be seen that the red and black lines are not quite parallel and that they

are less nearly so as the wire is brought nearer the face. The red lines (seen by the left eye)

seem to incline a little toward the right and the black lines (seen by the right eye) toward the

left. As the wire comes near and the convergence is great the horizontal lines will also show the

rotation. This apparent rotation of the lines is not, as in the case of the after-images, a sign that

the corresponding eye has rotated in the way that they have, but that it has rotated in the

opposite way. b. Repeat this with the head much inclined forward (the equivalent of elevating

the eyes) and with it thrown far back (equivalent of depressing the eyes) taking care that the

wire is always at the same distance from the eyes. In the first case the apparent rotation of th

[sic] lines is increased, and in the second decreased to zero or even transformed into rotation in

the opposite direction.

Cf. Helmholtz, op.cit. Fr 609 (468)-610 (470). Le Conte, Sight, 177-191. Hermann's Handbuch

der Physiol. III, Th. 1. 496 ff.

126. Involuntary movements of the eyes. Lay a small scrap of red paper on a large piece of

blue. Fixate some point on the edge of the red. After a few seconds of steady fixation, the color

near the line of separation, will be seen to brighten, now in the red and now in the blue. This is

due to the small unintentional movements of the eyes.

Footnotes

[1] For fuller information on rotation apparatus see the introduction to the section on color-

vision, to follow.

[2] My assistant Mr. T. L. Bolton, has noticed that these after-images are subject to illusory

transference like those of Ex. 118.

V. -- VISION. (Continued.)

SEEING OF LIGHT AND COLOR.

The aim of the following experiments is not to adjudicate conflicting color theories, but rather to

present the most important experimental facts that all color theories must regard.[1]

Authoritative accounts of the theories may be found as follows: Young-Helmholtz theory;

Helmholtz Handbuch der physiologischen Optik,[2] 2te Aufl., pp. 344-350. G

290-294, 320-

321, 367; F. 380-387, 424-425, 484. Also Popular Scientific Lectures, 1st Series, New York,

1885, pp. 249-256. Hering's theory; Hering Zur Lehre vom Lichtsinne, pp. 70-141, (two

communications to the Vienna Academy, April 23 and May 15, 1874); Ueber Newton's Gesetz

der Farbenmischung, pp. 76-79, a very brief account of his own in connection with a general

account of theories. These are the most prominent theories, and something on them, especially

on the first, will be found in the physiologies and in some works on the use of color in the arts.

Other theories more or less different from these will be found as follows: V. Kries: Die

Gesichtsempfindungen und ihre Analyse, Du Bois-Reymond's Archiv, 1882, Supplement-Band,

vi, 1-l78. Wundt: Physiol. Psychol., 2te Aufl.. pp. 453-456; 3te Auh., 491-496. Also Philos.

Studien, IV, 1888, 355-389. Donders: Ueber Farbensysteme Archiv für Ophthalmologie, XXVII,

1881, H. 1. Noch einmal die Farbensystem, ibid., XXX, 1884, 1. Göller: Die Analyse der

Lichtwellen durch das Auge. Du Bois-Reymond's Archiv, 1888. Christine Ladd Franklin, Eine

neue Theorie der Lichtempfindungen. Zeit. für Psychol., IV, 1892, 212.

On color vision in general may be mentioned, besides these works of Helmholtz, Hering and

Wundt: Fick: Qualität der Lichtempfindungen, Hermann's Handbuch der Physiologie III, Th. i,

pp. 160-232. Maxwell: On the Theory of Compound Colours [p. 391] and the Relation of the

Colours of the Spectrum, Phil. Trans. 1860; and On Colour Vision, Proc. Royal Institution of

Great Britain, VI; reprinted in Maxwell's Scientfic Papers, I, 410-440; II, 267-280. Rood:

Students' Text-book of Color, New York, 1881. Aubert: Physiologie der Netzhaut, Breslau,

1865; also Grundzüge der physiologischen Optik, Leipzig, 1878, pp. 479-572. This work forms

part of the second volume of Graefe und Saemisch's Handbuch der gesammten

Augenheilkunde. Charpentier: La Lumière et les Couleurs, Paris, 1888. Von Bezold: The

Theory of Color in its Relation to Art and Art Industry, Boston, 1876. Benson: Manual of the

Science of Colour, London, 1871, pp. xii, 58. Chevreul: The Principles of Harmony and

Contrast of Colors, London, 1859. LeConte: Sight, New York, 1881. Ladd: Elements of

Physiological Psychology, New York, 1887. Beaunis: Nouveaux Éléments de Physiologie

Humaine, Paris, 1888; and other standard physiologies.

Apparatus. In addition to the blue and red glass, the colored papers and the black and white

cardboard used in the previous section, there will be required for this, pieces of yellow, green

and violet glass, or of colored gelatine (see below), a small pane of clear glass, a mirror, a

sheet of white tissue paper or other semi-transparent paper, and pieces of gray paper or

cardboard of different intensities. Gray papers can be made by painting white paper over with

India ink; or a tolerable substitute may be made by overlaying black paper or cardboard with

one or more thicknesses of tissue paper.

For some of the contrast experiments (Ex. 141 ff.) the gummed parquetry rings and the lentil

dots used by the kindergarteners are extremely convenient, and are not expensive. The rings

are 1, 1 1-2 and 2 in. in diameter and 1-8 in. broad and are to be had in a considerable variety

of colors; the dots are 1-4 in, in diameter, and in six colors. See catalogue of the Milton Bradley

Co., Springfield, Mass., pp. 49 and 71.

The standard of color when exactness is important is, of course, the spectrum; and

experiments with pure (i.e., monochromatic) spectral colors are the final appeal. The apparatus

required for a complete study of color sensations with spectral colors, especially when

quantitative results are aimed at, is extremely refined and correspondingly expensive. The

spectrophotometer pictured on p. 355 of Helmholtz's Physiologische Optik, 2te Aufl., is quoted

by the makers, Franz Schmidt und Haensch, Stallschreiber-Strasse 4, Berlin, S., at mk. 750.

Other apparatus of similar purpose ranges from mk. 375 to mk. 3500. The spectrophotometer

of the Cambridge Scientific Instrument Co. costs £15. Simple qualitative experiments like those

of this section can, however, be made without very expensive apparatus, and for the most part

without spectral colors. Where spectral colors are employed a simple prism (costing from 15

cents upward), or at most an ordinary spectroscope, such as is found in every chemical and

physical laboratory, will serve amply. A pocket spectroscope even, costing from $5 or $6

upward, will show a good deal, and is useful in determining approximately the composition of

light transmitted by colored glass. If nothing more is done, it is desirable that the experimenters

familiarize themselves with the appearance of the spectral colors and the chief Fraunhofer lines

as landmarks in the spectrum. By combinations of thin sheets of colored gelatine, light that is

practically monochromatic can be secured; see a paper by Kirschmann, Ueber die Herstellung

monochromatischen Lichtes, Wundt's Philos. Studien, VI, 1891, pp. 543-551. Such sheets are

used before calcium lights in the [p. 392] theatre for the projection of colored lights upon the

stage end may be had of dealers in stereopticons. A. T. Thompson & Co. 13 Tremont Row

Boston, sell red, yellow, green, blue, violet and purple in sheets, 20 1-2 x 16 3-4 inches, at 25

cents a sheet. For many purposes these sheets are as good or better than colored glass.

For the study of the phenomena of color-mixing

with artificial colors, the most satisfactory

instrument is the color top or rotation color-mixer

in some one or other of its numerous forms. One

of these was mentioned in the introduction to the

previous section on the Mechanism of the Eye

and Vision in General, namely, a little electric

motor. All the experiments of this section that

require a color-mixer can be made with such a

one. Many it not all of them could be made with

the color tops sold as toys, or with the very

simple one suggested by Dr. Bowditch in his

Hints on Teaching Physiology, to wit, a button-mold fitted with a peg and spun with the fingers.

One made of a button-mold an inch and three quarters in diameter and carrying disks two and a

half inches in diameter, shows the contrast effects of Ex. 142d as elegantly se could be desired.

The disks me held in place by apiece of rubber tubing of very small bore fitting snugly upon the

stem and twisted down upon the disk like a nut. Larger apparatus specially designed for color-

mixing may be had of all physical instrument dealers. Among the rest may be mentioned the

Farbenkreisel made by R. Rothe, Mechaniker des physiologischen Instituts der k. k. Universität,

Prag (Wenzelsbad), at mk. 30. A fine instrument by the same maker for rotating a horizontal

disk either by toot or hand, with additional parts for studying the color-blindness of the

peripheral parts of the eye, costs mk. 160. The color-mixer of the Milton Bradley Co.,

Springfield, Mass., costs $10, including disks, etc.; the color-mixers of the Cambridge Scientific

Instrument Co., St. Tibb's Row Cambridge, England, cost £6-10 and £10. R: Jung, Heidelberg,

has rotation apparatus, including one that moves by clock-work at mk. 50-65 with disks. The

important thing in such a piece of apparatus is that it should rotate rapidly enough to give a

smooth and steady mixture of two colors when these occur but once each upon the disk, e.g.,

to give an even gray with a disk that has a solid 180° of black and a solid 180° of white; When

this is the case the two disks may be slipped together, as in the cut, and any required

proportion of the colors easily arranged. It the rotation is not sufficiently rapid the sectors must

be made smaller and more numerous, e.g., four sectors of black of 45° each separated by tour

sectors of white of the same size. This is not difficult when the proportions of color are to

remain constant, but where adjustments are to be made the multiplicity of sectors is a

disadvantage. Rothe and the Milton Bradley Co. supply colored paper disks in considerable

variety evenly cut, and this is an important point, for if the cutting is inexact the disks will appear

with bothersome fringes of color when in rotation. The centre hole in Rothe's disks is of course

cut to fit the Rothe color mixers. His disks may be had in two sizes, 20 cm. and 11 cm. in

diameter, at prices ranging from 60 to 105 kr. per doz. for the large, and 20 to 30 kr. for the

small, according to color. Colored papers of excellent color and surface (shiny papers are to be

avoided) may be had of R. Jung, Heidelberg. [p. 393] A rotation apparatus that will serve

excellently for class demonstrations is a carpenter's polishing lathe, which is to be obtained at

some hardware stores and sells at $4.50. It can be screwed to the table and worked by the

hand or foot. Unfortunately it can hardly he made to rotate rapidly enough to blend 180° of

white with 180° of black, but for fixed disks with more numerous sectors it works excellently;

and though made for so remote a purpose can be used without change and will carry disks up

to a foot in diameter. With a very little alteration it mould carry them twice as large. Perhaps a

maximum of simplicity is reached in the use of a boys' "buzzer" as a color-mixer which the

writer has heard recommended, but never tried. Special disks for use in certain experiments

are shown in cuts accompanying them.

In addition to the color-mixer and disks, a stereoscope end stereoscopic diagrams (see cuts

accompanying the experiments) and a double retracting prism will be needed. Any stereoscope

will answer but the hood and the central partition should be removed. The double refracting

prism may be purchased at no very great expense from dealers in physical instruments.

In Ex. 142b and 148b a smell wooden frame made by fixing two pieces of board 6x6 in.

together at right angles, is needed (see diagram accompanying Ex. 142b). The convenience of

the instrument is much increased if guide strips of wood or metal are attached to the vertical

and horizontal pieces, so that the diagrams to be used upon them will be held in place and yet

be easily interchangeable, For exhibiting a very deep black in Ex. 130a, a black box may be

prepared as follows. Make a light tight wooden box 8x10x12 inches in size; cut a two-inch hole

in one end and have the whole painted a dull black, both within and without. Before closing it

finally, divide it by a slanting partition running obliquely upward and forward from the back edge

of the bottom to a point on the top about four inches from the front. The front side of this

partition should be covered with black velvet. In comparison with the black that is seen on

looking through the hole against this slanting velvet, the gray character of the black paint, of

ordinary black cardboard, and even of black velvet, is easily recognized.

The easiest test for color-blindness is made with colored worsteds, which may be had of any

dealer in oculists' supplies. An approved selection of colors in sufficient variety is sold by N. D.

Whitney & Co., 129 Tremont street, Boston, Mass at $2.50. A small card of wools for testing

color vision is to be found on the inside cover of Galton's Life History Album, London,

Macmillan & Co., 1884.

Apparatus is helpful in measuring out the color fields In Ex. 128, though it need not be

elaborate. At its simplest two things are necessary: something for steadying the head, and a

broad surface perpendicular to the line of sight on which to map out the color fields. A block on

which to rest the chin and a convenient wall might do. If something a little more permanent is

desired, the headrest shown in the last section (American Journal of Psychology, IV. 1891-82,

474) can be clamped to the front edge of a narrow table, and a screen of light boards (or better,

a wooden frame covered with black or gray cloth) fastened about a foot back from it. This

distance must not be great, or the screen will need to be of excessive size. It is well to paste a

vertical and horizontal scale upon the screen crossing at the point immediately before the eye,

so that distances may at once be read off. Such an instrument is known as a campimeter. A

more perfect instrument for this purpose is the perimeter. Of this instrument there are many

forms; that of Priestly Smith is perhaps as convenient as any except the most elaborate [p. 394]

ones. In this instrument, not to attempt a detailed description, the place of the screen is

supplied by a curved arm that can be turned about an axis at the point on which the eye is

fixed, and in turning would describe a hemisphere of which the eye is the centre and the fixation

point the pole. The arm is marked for every 5° and the limits of the field of vision on any

meridian can at once read off and recorded. The record is made by a needle prick in a diagram

carried by the instrument, a new diagram being inserted for each eye tested. The price of this

instrument from R. Jung Heidelberg, is mk. 60, from New York dealers $30.

For Ex. 145 end 151 an apparatus devised by Hering and described by him in the Zeitschrift für

Psychologie, I 1890, 23-28, is extremely convenient. It is made by R. Rothe of Prag at 28

marks. The apparatus is simple, however and any carpenter can make of wood one that will

answer. The first aim of this instrument is to secure a binocular mixture of blue and red. For

that purpose blue and red glasses before the eyes may be used, provided that a good deal of

white can also be mixed in with the color of the glass. This is done by letting the glasses stand

at an angle before the eyes and reflect on their upper surface the images of suitably placed

white screens. The quantity of white light is regulated by the position of the screens with

reference to the source of illumination and by the inclination of the colored glasses. The

following cut shows diagrammatically what the arrangement of glasses and screens is.

In the cut W

and W

are the screens just

spoken of R and B the red and blue glasses,

W a white surface carrying a narrow black

strip at s, and k is the point upon which the

eyes are fixed. In an instrument made by a

carpenter for the laboratory of Clark

University, the following plan was followed; it

is here reproduced not because it is the best,

but for the sake of definiteness. The stuff

used in the instrument was almost all seven-

eighths of an inch thick, and that thickness

may be understood except where something

else is stated. The base is a board 30 in.

long;, 12 in, wide. In the middle of this is

placed another board 12 in. long and 10 in, wide, leaving a margin of an inch on each side and

of nine inches at the ends. This little platform bears the white cardboard corresponding to W in

the diagram. On the nearer edge of this platform is fastened an upright post 15 in. high, 3 in.

wide. At its upper end on the forward side this post carries the frames that hold the glasses R

and B. The glasses are 4 in. square, and are framed on three sides only, the upper edge being

left tree so that the glasses may come close to the eyes. The frames are small pieces of board

8 in, long, 5 in, wide with a square piece (three and three-quarters inches on the side) taken

from the middle of their upper ends, leaving them like U with very square corners and a heavy

bottom. Over these square holes the glasses are fastened. The frames are fastened with a

single screw each to the post, the screws penetrating the frames about an inch and a half from

the free edge of the glass. When in position, the glasses rise about three-quarters of an inch

above the top of the post, and stand like the sides of a roof. They do not quite meet, however

but leave a space for the observer's nose between them when the apparatus is in use. The

screws that hold [p. 395] the frames should be tight enough to hold the frames in position, but

not so tight as to prevent their turning in adjustment. On the front of the post and about six

inches upward from its foot is a wire about three and a half inches long extending forward from

the surface of the post and perpendicular to it. At its end is a little button of cork, the fixation

point k in the diagram. The side screens of the instrument are exactly alike and the description

of one will do for both. Each screen is a piece of half -inch board 9 in, wide and 13½ in. long.

This board turns midway from top to bottom on a horizontal axis and in a light frame just large

enough to enclose it. The frame itself is fastened to a broad piece of board which forms its base

and rests in turn on the base board of the instrument. A peg in the middle of the first of these,

fitting into a hole in the last, allows the rotation of the frame and screen about a vertical axis.

The screen is thus made adjustable in any direction. Its face is covered with white cardboard.

The only remaining part of the instrument is the strip of black paper, a quarter of an inch wide,

represented by s in the diagram, which is pasted on W perpendicular to the post. It is highly

Important that W be without speck or spot, and that the colored glasses be as free from flaws

as possible. The instrument as described is intended for binocular contrast. For binocular color-

mixing, other pieces of glass besides red and blue are needed for other combinations and the

black strip is not required.

Another simple demonstrational instrument of Hering's contriving is for the study of changes of

brightness in colors and can also be adapted for contrast. Its principle is the same as that used

in the side screens in the last instrument, namely, change of position with reference to the

source of illumination. A white card, provided that its surface is not shiny, receives a maximum

of light and looks brightest when it stands perpendicular to the light. As it is turned and the light

falls obliquely upon it, it receives less and less and looks darker and darker. If it is shiny, as

most paper and cardboard are, this change is not uniform, but this does not much interfere in

this instance. The paper used should however, be dull finished. The instrument at its simplest is

a tall box open in front and with a hole in the top to look through. It is painted black inside and

contains a screen that can be turned about a horizontal axis, and thus receive light

perpendicularly or obliquely as desired. It is, however, convenient to have a frame instead of a

permanent screen so that, a number of cards of different color or brightness can be

interchanged, and to have the box double so that two frames can be used side by side and

comparative tests can be made. When contrast is to be introduced, a second pair of frames

above the first and high enough so as not to shade them are introduced. The cards that are

used in these upper frames must each be pierced with s hole, say 2x4 cm., near the middle, in

such a position that when the eye looks through them from the top of the box, nothing but the

card in the frame below can be seen. The hole must be carefully and cleanly cut, and the edge,

if it shows white, must he colored like the surface of the card. When such a hole is looked at

with a single eye, it is easy to conceive the part of the lower card seen, not to be really below,

but a part of the upper card itself. This illusion might be strengthened by the use of a feebly

convex lens to exclude exact accommodation. Changes of the inclination of the upper frame

(provided there is no reflection from its undersurfaces) can produce no real change in the

illumination of the lower one, but very striking changes seem to follow such changes of the

upper one. This instrument in finished form, though without the additional [p. 396] frames for

contrast, can be had like Hering's other apparatus of his mechanic, R. Rothe, Prag.

Many of the color experiments to follow can be demonstrated before a considerable audience

by the use of a projection lantern, and some makers of lanterns have diagrams for contrast

colors, etc. Their preparation, however, can offer little difficulty to those familiar with the use of

the lantern and with the ordinary forms of the experiments.

A given color sensation may be changed in three ways: in color-tone, in intensity and in

saturation, or to use Maxwell's terms, in hue, shade and tint. Changes in color-tone are such as

are experienced when the eye runs through the successive colors of the spectrum. Changes in

intensity are changes in the brightness of the color. Changes in saturation are such as are

produced by the addition of white; when much white light is added, the color is a little saturated.

Changes in intensity and saturation if excessive involve some change of color-tone also. The

number of primary colors is various in various theories; red, green and violet (or blue) are

selected by the supporters of the Young-Helmholtz theory, red, green, yellow and blue by

Hering Mach and others, while Wundt is indisposed to make any particular ones more original

for sensation then the rest.

127. Color-blindness. Holmgren's method. Spread the worsteds on a white cloth in good

daylight. Pick out a light green (i.e., a little saturated green) that leans neither toward the blue

nor the yellow; lay it by itself and require the person to be tested to pick out and lay beside it all

other skeins that are colored like it, not confining himself, however, to exact matches, but taking

somewhat darker and lighter shades also, so long as the difference is only in brightness and

not in color-tone. Do not tell him to pick out "the greens" nor require him to use or understand

color words in any way; simply require the sorting. If he makes errors, putting grays, light

browns, salmons or straws[3] with the green, he is color-blind; if he hesitates over the

erroneous colors and has considerable difficulty his color-vision is probably defective, but in a

less degree. If the experimentee makes errors, try him further to discover whether he is red-

blind or green-blind by asking him to select the colors, including darker and lighter shades, that

resemble a purple (near magenta) skein. If he is red-blind, he will err by selecting blues or

violets, or both; if he is green-blind, he will select green or gray, or both, or if he chooses any

blues and violets, they will be the brightest shades. It he makes no errors in this case after

having made them in the previous case, his color-blindness is incomplete. Violet blindness is

rare. Complete certainty in the use of even such a simple method as this is not to be expected

without a full study of the method and experience in its application.

On color-blindness and methods of testing for it cf. Helmholtz: Op. cit. G

357 372[sic]. 456-

462; G

294-300, 847-848; F. 388-400. Jeffries: Color blindness, its dangers and its detection,

Boston. 1879 (this work contains a seventeen-page bibliography on color-blindness and kindred

topics); also an article on Color-blindness in the Reference Handbook of the Medical Sciences,

New York, 1886 II, 241. Rayleigh and others: Report of the Royal Society's Committee on

Colour Vision. Proc. Royal Soc., LI, no. 311, July 19, 1992. Hering: Zur Diagnostik des

Farbenblindheit, Archiv für Ophthamologie, XXXVI, 1890, Heft I, 217-233; also Die

Untersuchung einseitiger Störungen des Farbensinnes mittels binocularer Farbenleichungen.

Archiv f. Ophtal., XXXVI, 1890, H. 3, 1-23. See also a [p. 397] paper by Hess in the same

place, pp. 24-36. Kirchmann: Beiträge zur Kenntniss der Farbenblindheit. Wundt's Philos.

Studien. VIII, 1892, Heft 2 i. 3; Helmholtz, Hering, Kirschmann and others give exact methods

for determining the particular colors that are lacking. On differences in the apparent extent of

the spectrum in different observers, see Morgan: Animal Life and Intelligence. pp. 280-283.

128. Vision with the peripheral portions of the retina. a. Campimetry. Color-blindness is normal

on the peripheral portion of the retina. At the very centre the pigment of the yellow spot itself

interferes somewhat with the correct perception of mixed colors containing blue (cf. Ex. 110). In

a zone immediately surrounding this all colors can be recognized. Outside of this again is a

zone in which blue and yellow alone can be distinguished, and at the outermost parts not even

these, all colors appearing black, white, or gray. The zones are of course not sharply bounded,

but blend into one another, their limits depending on the intensity and area of the colors used.

a.[sic] With the campimetrical apparatus at hand, find at what angles from the centre of vision

on the vertical and horizontal meridians of the eye the four principal colors, red, yellow, green

and blue can be recognized; try also for white. Keep the eye steadily fixed on the fixation mark

of the instrument and have an assistant slide the color (say a bit of colored paper 5 mm. square

pasted near the end of a strip of black cardboard an inch wide) slowly into the field from the

outside. It will be well to move the paper slowly to and fro at right angles to the meridian on

which the test is made, so as to avoid retinal fatigue. Take a record of the point at which the

color can first be seen with certainty. Repeat several times and average the results. The size of

the colored spot shown should be constant for the different colors, and the background

(preferably black) against which the colors are seen should remain the same in all the

experiments. b. Repeat the tests with a colored square 10 mm. on a side, and notice the earlier

recognition of its color as it approaches from the periphery. c. Try bringing slowly into the field

(best from the nasal side) bits of paper of various color, especially violet purple, orange,

greenish yellow and greenish blue; or better, hold the bit of paper somewhat on the nasal side

of the field and turn the eye slowly toward it, beginning at a considerable angle from it. If the

paper is held before a background containing a line along which the eye can approach the

paper, the eye will be assisted in making its approach gradual. Observe that on the outer parts

of the retina these colors first get their yellow or blue constituents, and only later the red or

green, and appear in their true color. If the range of choice is sufficiently large it may be

possible to find a red (inclined toward red purple), a green (inclined toward the blue), which, like

pure blue and yellow, change only in saturation and not at all in color tone as they move inward

toward the centre of the field. These four colors are the Urfarben or primary colors of Hering.

Helmholtz: Op cit.. G

372-374 F. 398-400. Hess: Ueber drn Farbensin bei indirektem Sehen.

Archiv für Ophthalmologie. XXXV, 1889, H. 4. Hering: Ueber die Hypothesen zur Erklärung der

peripheren Farbenblindheit. Arch. F. Ophth. XXXV, 1889 H. 4. pp. 63-83; XXXVI, H. 1, 264.

Fick: Zur Theorie des Farbensinnes bei indirektem Sehen, Pflüger's Archiv, XLVII, 1890, 274-

285. Aubert, Phys. Opt., 539·546.

129. Changes in color tone. With spectral lights, change of vibration rate, if not too small,

means change of color tone, but equal changes in vibration rate do not involve equal changes

in color tone. The change of color tone is most rapid in the green region of the spectrum, less

rapid at the red and violet ends. a. With the spectroscope and daylight and the characteristic D,

E, F and H lines. The D line lies in the golden yellow, E the green, F in the blue, and H at the

end of the violet. Between the D line and [p. 398] the F line, the vibration rate changes from 526

to 640 billion vibrations per second, and the color runs from yellow, through green to blue, while

from F to H with the greater change in vibration rate from 640 to 790 billion per second, the

change is only from blue to violet. c.[sic] Notice the tendency of the succession of spectral

colors to return upon itself, shown in the resemblance of the red and violet.

Helmholtz: Op. cit., G

289, 320. G

237, F. 319. Rood: Op. cit., 27. Wundt: Op. cit., 1, 429.

Fick: Op. cit., 173-175, 183. Aubert. Phys. Opt., 530.

130. Changes in intensity. Black and white. Black and

white are the extremes of intensity in the series of

grays. The ordinary black and white of conversation

are considerably short of these extremes. a.

Compare a bit of black velvet or black cardboard with

the black of the black box described above. b.

Compare ordinary white paper in diffused light with

the same in direct sunlight or with a brightly

illuminated white cloud. c. Just observable

differences with medium intensities. Prepare a disk

like that shown in the accompanying cut by drawing

along a radius of the disk a succession of short lines

of equal breadth. Let the breadth of the line

correspond to about one degree on me edge of the

disk. Since the breadth of the line is everywhere the

same, it will occupy a relatively greater portion as it

nears the centre. When the disk is set in rapid rotation, each short line will give a faint gray ring,

those at the outer edge being very faint, those nearer the centre darker. Find which is the

faintest ring that can be seen, and calculate the proportions of black and white in it.[4] The ratio

of the black to the white measures approximately the just observable decrease in intensity

below the general brightness of the disk. The results of Helmholtz and Aubert are respectively:

Helmholtz, 1:117 to 1:167. Aubert, 1:102 to 1:186, the differences depending on the intensity of

the general illumination of the disk. Some wandering of the eyes is helpful, but too rapid

motions of the eyes, which tend to break up the even gray of the rings, must be avoided. It is

absolutely essential to have the rotation very rapid and perfectly free from vibration -- so rapid

that with the moderate motions of the eyes, the uniform gray of the rings is not disturbed. If

great rapidity is impossible replace the single black line by two of proportionately less breadth

on opposite sides of the disk, or by four at 90°.

Helmholtz: Op. cit., G

384-393, G

, 310-316, F. 411-419. Aubert: Physiol. Optik., 489-492.

131. Changes in intensity. Colors other than black and white. At their maximum intensity, all

colors tend toward white or yellowish white, green becoming first yellow and then white, red

progressing hardly beyond the yellow, but blue and violet easily reaching white. [p. 399] a. Fix a

prism in the sunlight so that it projects an extended

spectrum on the wall. Hold a card, pierced with a pin-

hole before the eye, and bring the eye successively

into the different colors, looking meanwhile through

the pin-hole at the prism. Something of the same kind

may be seen by looking through pieces of colored

glass at the disk of the sun behind a cloud (in which

case the portions of the cloud seen at the sides of the

glass afford a means of comparison), Or at the image

of the sun reflected from an unsilvered glass plate, or

by concentrating light from colored glass on white

paper with a convex lens. b. It is easy to reduce the

intensity of colors with the color-mixer by spreading

the light of a colored sector over the whole surface of

a disk otherwise black.[5] Make a succession of

mixtures of red and black on the color-mixer, beginning with a proportion of red that makes a

barely observable change and increase the proportion till the red decidedly predominates in the

mixture. Place a smaller disk of black over the larger disks so as to have a standard black in

the field. If any of the red shows through either black disk several of the latter should be used

together to prevent it. Try also with the other chief colors. Disks like the diagram (in which

shaded parts stand for color and solid black for black) show the whole series of such gradations

at once though not quite so satisfactorily. c. Carry a number of small slips of colored paper into

a darkened room, or look at them through a fine needle hole in a card, and notice the order in

which they lose their color quality. d. Adjust the spectroscope so that the chief Fraunhofer lines

can be seen, and then gradually narrow the slit through which the light enters the instrument.

Observe that red, green and violet-blue with a trace of yellow persist longer than the

intermediate colors, and that when all the color is gone, there still remains some light in the

region of the green. This experiment must be performed in a dark room, or the observer must

envelope his head and the ocular of the instrument with opaque cloth. e. Purkinje's

phenomenon. In a light of moderate brightness, choose a bit of red paper and a bit of blue

paper that are about equal in intensity and saturation, or better, make such a pair with the

color-mixer by adding white or black till the intensity and saturation appear the same. Carry

both into the full sunlight and notice which appears brightest. Carry both into a darkened room,

or observe them in deep twilight, or through a very fine needle hole in a card, or even with

nearly closed eyes, and again notice which seems brightest. Cf. also Ex. 142a.

Helmholtz: Op. cit., or G

402-444; on a, 284-285, 322-324.,465, 466; on b, 469, 471-472; on e,

428-430, 443-444. G

, 234, 280, 281, 317-321. F 315, 369, 370, 420-425. Fick: Op. cit., 200-

202. Aubert. Phys. Opt.. 531-536. Rood: Op. cit., 181-194. For measurements of the just

observable differences of intensity for different colors, see Helmholtz: Op. cit., G

402-416;

Aubert, Phys. Opt., 531; Fick: Op. cit., 177; and the references given by them. Benson: Op. cit.

132. Changes in saturation. Repeat Ex. 131b, using colored sectors or stars on white disks

instead of black. If star disks are used, it is best to give the rays of the star a leaf shape, for the

smaller quantities of color toward the outer ends of the narrow rays fail to [p. 400] make can

impression on the white. Notice the paling of the color when mixed with white and relatively

preponderating effect of the latter. Notice also a tendency to change in color tone as well as in

saturation, especially when the amount of color is small. Red tends toward rose, orange toward

red, indigo toward violet, blue-green toward blue, etc. According to Rood's experiments with the

color-mixer, yellow-green and violet are unchanged; Helmholtz with spectral colors gets

somewhat different results.

Helmholtz: Op. cit. G

322, 476-471. G

281, F. 369. Aubert: Phys. Opt., 531-532. Rood: Op.

cit., 194-201.

133. Size of the colored field. The color sensation is not independent of the size of the retinal

area stimulated, if the latter is small; and is also affected by the background against which the

small colored area is seen. Paste on pieces of black and white cardboard small squares of

several kinds of colored paper, one series 5 mm. square, one 2 mm. square and one 1 mm.

square. Walk backward from them and notice their loss of color. Observe also the changes in

color-tone.

Helmholtz: Op cit., G

374-375, G

300, F. 399-400. Aubert: Phys. Opt., 536-539. E. Fick: Nitiz

über Farbenimpfindung, Pflüger's Archiv. XVII. 1878, 152 153·

134. Some Phenomena of Rotating Disks. Talbot-

Plateau Law. In several experiments of this section use

has been made of rotating disks in studying colors and

color combination. All such use depends on the

phenomenon of positive after-images (Cf. Ex. 116,

Amer. Jour. Psychol., IV, 1891-92, 486). A disturbance

set up in the retina does not at once subside but lasts

an instant after the removal of the stimulus. If stimuli

follow in sufficiently rapid succession the disturbances

are added to one another and fused and the result is the

same as though the stimuli had reached the retina

simultaneously. The rate of succession necessary to

give a uniform sensation is from 25 to 30 per second,

the rate depending on the illumination of the disk, the

higher rate being required for the greater illumination.

When once this uniform sensation has been reached

the color and brightness of any given concentric ring of the disk are the same that it would have if

all the light reflected from it were evenly distributed over its surface, and no further increase in

rapidity produces any effect upon its appearance. This is the Talbot-Plateau law. Rotate a disk like

that shown in the cut, increasing the rapidity till the innermost portion gives a uniform gray. When

this occurs, the rate of recurrence in the outer ring is 32 times more rapid than in the innermost,

and yet no difference in shade is to be seen. To show that the gray is actually of the same

brightness that would come from an even distribution of the light reflected from the whole surface

of the ring, look at the disk when at rest through a double convex lens held at such a distance

from the eye and disk that no distinct image is termed, but the disk looks an even blur of gray.

When the disk is put in rapid rotation the gray remains unchanged. [p. 401]

On rotating disks and their phenomena in general cf. Helmholtz, op. cit., G

480-501, G

337-

355, F. 455-471. On the Talbot-Plateau law cf. Helmholtz, op. cit., G

482-483, G

338-340. F.

446-450. Aubert, Phys. Opt., 515-516.

135. Some Phenomena of Rotating Disks. Brücke's experiment. As the disk used in the last

experiment is allowed gradually to go slower and slower, there will be observed in one ring after

another, beginning with the inner one, just as it loses its uniform character, a notable

brightening. The white sectors now have opportunity to produce their full effect upon the retina

before they are succeeded and their impression cut off by black sectors.

Helmholtz, op. cit., G

481-485, G

338-341, F. 446-450. Aubert, Phys. Opt., 510.

136. Some Phenomena of Rotating Disks. The Münsterberg-Jastrow phenomenon. a. When

the disk used in the last experiment gives a uniform gray, pass rapidly before it a thin wooden

rod or thick wire, and notice that a multitude of shadowy images of the rod will appear on the

disk. The number of images is greatest in the portion of the disk having the most frequent

interchange of black and white. b. Exchange the disk for one carrying two or more colors.

Notice the repetition of the phenomenon, and that the colors of the images are the colors

(otherwise completely blended) which the disk actually carries. The explanation of the

phenomenon is not altogether clear, but the sudden changes of the background against which

the rod is seen seem to have an effect not unlike that of a stroboscopic disk or of intermittent

illumination, which would show the rod at rest in its successive positions.

Jastrow: A Novel Optical Illusion. Amer. Jour. Psych., IV, 1891-92, 201-208.

137. Some Phenomena of Rotating Disks. Fechner's colors. Relate the disk used in Ex. 119 or

that used in 134, or indeed almost any black and white disk with a less rapidity than that

required to give a uniform gray keeping the eyes from following the motion of the disk, and

notice the play of colors on its surface. These vary with the rate of the disk and the intensity of

the illumination. The colors may not at once appear, but are not difficult to get with steady and

attentive gazing. The colors owe their existence to an analysis of the light of the white sectors,

depending, not on the different wave lengths of the colors, as in the case of the prism, but on

the differences in the times at which the different primary color sensations reach their maximum

in sensation. The intermittent stimulation causes a rise and fall in the intensities of the

fundamental sensations, but the instant of greatest excitation is not the same for all.

Helmholtz. op. cit., G

530-532, G

380-381, F. 500-502. Aubert, Phys. Opt., 560.

138. Color-mixing. A general law of color-mixing, and one upon which almost all experiments

with artificial colors must depend, may be stated as follows: Like appearing colors produce like

appearing mixtures.[6] Thus an orange that is mixed from red and yellow spectral lights will

produce the same purple when mixed with violet that spectral orange of like intensity would

itself produce. The colored papers with which the experiments below are made are very far

from simple colors (as can easily be seen by looking at scraps of them on a black background

through a prism), yet they produce the same mixtures that spectral colors of equal tone,

intensity and saturation would do. Three colors properly selected [p. 402] from the whole range

serve to produce by their mixtures all the intermediate colors (though generally in less

saturation) besides purple and white (i.e., gray). The colors generally selected are red, green

and violet. Green cannot be mixed from colors that themselves do not resemble it, i.e., it can be

mixed from yellow-green and blue-green, but not from yellow and blue, and not in anything like

spectral saturation. a. Mix a yellow from red and green on the color-mixer. The yellow produced

will be dark, and as a test of its purity should be matched with a mixture of yellow and black

made with smaller disks set on above the first. In the same way mix a blue from green and

violet that shall match a mixture of blue and black. b. from red and violet or blue, mix several

shades of purple between violet and purple. c. From red, green and violet, mix a gray that shall

match a mixture of black and white on the small disk.

For demonstrational purposes the result of mixing two colors in different proportions can be

shown on a single disk of the star form (see Ex. 131) by painting the star in one color and the

ground of the disk in another (or by pasting colored papers instead of painting), but in either

case some trial will be necessary to determine the proper size for the rays.

Helmholtz, op. cit., G

311-316 320-322, 325-323, 350- 357, 375-376, 473, 485, G

272-277,

279-281, 282, 341, F. 359-365, 367-369, 450. Aubert, Phys. Opt., 521-524, 527-528.

139. Complementary Colors. The combination of red, green and violet mentioned in the last

experiment is not, the only combination that mill give white or gray. For every color there is

another or complementary color, which mixed with it will give a colorless combination. Some of

these pairs are red and blue-green, yellow and indigo blue, green and purple, blue and orange-

yellow, violet and yellow-green. a. Try several of these pairs upon the color-mixer, matching the

resultant gray carefully with a mixture of black and white on the small disk. It will probably be

found in some cases that no possible proportions of the co papers at hand will give a pure gray.

In that case a little of the color complementary to that remaining in the gray must be added.

Suppose the red and blue-green papers give, when combined, gray with a tinge of brown (i.e.,

dark orange), a certain amount of blue or indigo must be added to compensate. For example,

with certain papers 180° of blue-green, +36° indigo +144° red make a gray that matches 90°

white +270° black. To see the true complement of the red used it is then necessary to prepare

a disk carrying green and indigo only in the proportions of 180 and 36, i.e., 300° blue-green. 60°

indigo. In the same way the complement of the blue-green used is a bluer red than that of the

red paper, and may be seen by itself by mixing 288° red with 72° indigo. It is very important

here and in all cases where a resultant white or gray is to be observed, to have some

undoubted white or gray in the field to prevent illusions over very faint tinges of color. b.

Negative after-images when projected on a colorless gray or white surface are seen in colors

complementary to those that give rise to the after-images. Compare a pair of complementary

colors found in this way with the same pair as found on the color-mixer.

Helmholtz. op. cit., G

316-319, G

277-278. F. 365-367. Aubert, Phys. Opt., 521-524.

Complementary colors can be well seen with polarized light. See pictures of the schistoscope,

an instrument for showing them by this meant in Rood, op. cit., 162; description of Rose's

chromatometer (Farbenmessser). Helmholtz, op. cit., F 397; also the Leukoskop, Helmholtz,

op. cit., G

368, and references there given.

140. Other Methods of Mixing Colored Lights. a. The

simplest of these methods is by reflection and

transmission. The colors to [p. 403] be mixed are

pieced on a horizontal surface on opposite sides of a

vertical glass plate. The eye is brought into such a

position that the reflected image of the colored held

on the eye side appears to overlie the held on the

other side seen through the glass. The glass must of

course be of good quality and clean. The relative

intensity of the colors can he varied by varying their

distance from the glass. Bringing the colors near the

glass or raising the eye, strengthens the reflected and weakens the transmitted light. Strips of

paper placed with their ends next the glass will show an even blending from a mixture in which

one predominates to one in which the other predominates, provided the illumination is equal. To

mix two colors in exactly equal proportions, arrange them with black and white, as in the

diagram below.

Adjust the glass till the grays made by the black and white at the ends exactly match; the colors

will then be mixed half end half.

By substituting a bit of glass on a black background for one of the colors and then placing the

instrument so that a portion of clear sky may be reflected in the glass, it is possible to mix sky

blue with its complement, or with any other color.

b. Colored areas placed side by side can be mixed with the aid of a double retracting prism.

The prism doubles both fields and causes a partial overlapping. In the overlapped portion the

colors are mixed.

c. Spectral colors can be mixed though in an inexact way without more apparatus than a prism

and a piece of black cardboard. Fit a piece of black cardboard into the window frame so that it

shall cover one pane completely. Cut in the middle of it two narrow slits (1-2 mm. wide and 10

cm. long), meeting each other at right angles and making a broad V. The cuts should be clean

and sharp and the slits of uniform width. Look at this V from a distance 10 or 12 feet, holding

the prism vertical. Each arm of the V will give a spectrum, and where they cross. some spots of

mixed color may be made out, especially the red and violet giving purple, and red and green

giving yellow. The early studies of Helmholtz were [p. 404] made with apparatus arranged on

this principle, but more refined. If lines finer than can conveniently be out in the cardboard are

desired, they can easily be made (after a suggestion of Prof. Pickering) by tracing them on a

piece of smoked glass. If the sunlight is allowed to fall on a prism and the spectrum is caught

on a white wall or a screen, colors may be mixed with a double refracting prism like the colored

fields mentioned above.

d. Something may be done in the way of mixing colored lights with a prism and narrow stripe of

paper or a diagram like Plate I, or still better, a similar diagram in which black takes the place of

white and vice versa. Since a prism refracts different kinds of light to different degrees, it

produces a multitude of partially overlapping images or a bright object, which appear to the eye

as colored fringes. (Observe through a prism a square inch of white paper on a black

background.) These overlapping images may be illustrated by the following diagram, in which

the horizontal lines stand for the images and the capital letters for the colors of light producing

them.

In the area a b c d all the images overlap and the white of the paper is still seen. Toward the left

from a, however the different kinds of light gradually fail, beginning with the red. The successive

colors from greenish blue to violet result from the mixture of what remains. At the other end a

similar falling away of the colors the succession from greenish yellow to red. In Fig. 1, Plate I,

spectra seen on the upper and lower edges of the square are brought side by side; on one side

red, orange and yellow, and on the other greenish blue, blue and violet. The colors that stand

side by side are complementary pairs both in color tone, intensity and saturation for the

greenish blue is the white or the paper less the red, and the blue the same less the red, orange

and yellow, and so with the rest; and if the two spectra could be exactly superposed they would

make precisely the white from which they originated.

If a very narrow strip of white upon a black ground is looked at through the prism, the images

overlap less and another color appears, namely, green, as may be seen in Fig. 2 on the narrow

white band between the black bars. When, on the other hand, a narrow black band on a white

ground is taken, the spectra of the white surface above and that below partially overlap and

give another set of mixtures. If the diagram is held near the prism at first and then gradually

withdrawn from it, the advance and mixing of the spectra can easily be followed. Besides the

greenish yellow at one end and the greenish blue at the other, there are a rich purple,

complementary to the green beside it, and a white between the purple and the greenish yellow.

The last is a white produced by the mixture of the blue of one spectrum with the

complementary yellow of the other.

In Fig. 3 are shown a number of color mixtures with different proportions of the constituents. In

the spectra from the white [p. 405] triangle appear mixtures of each color in the spectrum seen

on the white band in Fig. 2, with every other color found there. Upon the black triangle in the

spectra from the white edges above and below are seen mixtures similar to those on the black

band in the same figure. The diagram should be at such a distance from the prism that a little of

the white and black triangles can yet be seen.

On methods of color mixing cf. Helmholtz. op. cit., G

350-357, 485, 491-493; G

303-306, 346-

349; F. 402-407, 450, 457-461. Aubert. Phys. Opt., 521-524,. Maxwell, op. cit. On a and d

Benson, op. cit.

141. Contrast. The effect of one color on another when not mixed with it, but presented to the

eye successively or in adjacent fields, is known as contrast. Two kinds are distinguished,

Successive contrast and Simultaneous contrast. The color that is changed or caused to appear

upon a colorless surface, is known as the induced color; the color that causes the change is

celled the inducing color. Successive contrast is largely a matter of negative after-images, and

their projection upon different backgrounds. Successive contrast: a. Prepare a set of colored

fields of the principle colors, including white and black, say 3x5 inches in size, and some small

bits of the same colors, say 1 cm. square. Lay a small square on the black held, get a strong

negative after-image and project it first on the white and then on the other fields. Notice that the

color of the after-image spot is that of the held on which it is projected minus the color that

produced the spot; e.g., the after-image of red projected on violet looks blue and on orange

looks yellow. Or, to say the same thing in other words, the color of the spot is a mixture of the

color of the after-image with the color of the ground upon which it is projected. Thus the blue-

green after-image from the red, when mixed with violet, gives blue; when mixed with orange

gives yellow. Notice that when the image is projected upon a field of the same color it causes

the spot on which it rests to look dull and faded but when it is projected upon a field of

complementary color it makes the spot richer and more saturated. In general, colors that are

complementary or nearly so are helped by contrast, those that resemble each other more

nearly are injured in appearance. b. These effects in even greater brilliancy can be seen by

laying the small square of color directly on the larger colored surface, staring at it a few

seconds and then suddenly puffing it away with the breath. Cf. also Ex. 126. c. This contrast

effect may be so strong as actually to overcome a moderately strong objective color. Place a

small piece of opaque orange paper in the middle of a pane of red glass and look through the

glass at a clear sky or bright cloud. The strength of the induced blue green will be sufficient to

make the orange seem blue. d. The contrasting color may even be made to appear upon a

surface faintly tinged with inducing color. Rotate one of the disks used in Ex. 132 or Ex. 131b

rapidly enough to produce an even mixture. Bring the eye within six or eight inches of the disk,

stare steadily at the centre for ten or twenty seconds; then suddenly draw back the head. The

complementary color will appear to rush in upon the disk from all sides. The explanation is that

after the withdrawal of the head the retinal image of the whole of the disk rests upon the part

before fatigued by the intensely colored center of the disk.

Helmholtz. op. cit., G

537-542, G

388-392, F. 510-515.

142. Mixed contrasts. When special precautions are not taken to exclude successive contrast,

both kinds co-operate in the general effect. Some of the results are striking and beautiful. a.

Colored [p. 406] shadows. (1) Arrange two lights so that they shall cast a double shadow of a

pencil or small rod upon a white surface, and regulate the brightness (or distance) of the lights

so that the shadows shall be about equally dark. The daylight will answer for one light if it is not

too strong, but it must come from an overcast sky, for the from a blue sky is itself blue.

Introduce different colored one after another before one of the lights and notice the beautiful

complementary color that immediately appears in the shadow belonging to that light. Cf. also

Ex. 144. (2) Use a blue glass and adjust the relative intensities of the lights so that the yellow

shadow appears at its brightest, and notice that it seems as bright as or brighter than the

surrounding blue. As a matter of fact, however, it receives less light than the surrounding

portions, for as a shadow it represents the portion of the held from which the light is partly cut

off.

b. Mirror contrasts. (1) Ragona Scinà's experiment. Place upon the horizontal and vertical

surfaces of the frame described above, white cards carrying black diagrams. Any black spot will

answer but for this experiment diagrams made up of sets of heavy concentric black rings (lines a

quarter of an inch wide) separated by white rings of triple width, give an excellent effect. The

diameters should be so chosen that a black ring on the horizontal diagram shall correspond to a

white one on the vertical and vise versa, and shall appear to lie in the midst of the white when the

diagrams are combined in the way immediately to be described. A pair of diagrams made up of

parallel black bars, a quarter of an inch wide, separated by quarter inch spaces, and so placed in

the instrument that they give a checker-board pattern when combined, are useful for keeping in

the field a true black with which the changed colors can he compared. The diagrams being in

place, hold between the two at an angle of 45° a pane of colored glass, say green, and observe

that the black of the horizontal diagram seems tinged with the complementary color that is, purple.

This contrast color may often be improved by slightly altering the inclination of the glass, or by

changing the relative illumination of the diagrams by interposing a colorless screen between one

or the other of them and the source of light, or by shifting the whole instrument. The mechanism of

this experiment will be readily understood after a consideration of the accompanying cut. The

glass plate is represented by CD, the black portion of

the vertical diagram by the projection opposite A, that

of the horizontal diagram by the projection at B. The

light reaching the eye from the white portion of the

horizontal diagram is colored green by the glass; that

from the white portion of the vertical diagram is

reflected from the upper surface of the plate, and is

therefore uncolored[7]. The mixture of the two gives a

light green field. For simplicity, we may assume that

no light comes from the black portions of the

diagram. In the portion of the light green field

corresponding to the black of the vertical diagram,

the white component will be wanting and the green

will appear undiluted; in the portion corresponding [p.

407] to the black of the horizontal diagram, the green

component will be wanting and the faint white (i.e.,

gray) should appear by itself. It does not, however,

because of the contrast color induced upon it. As a

matter of fact, the black portions are not absolutely

black; the small amount of light that comes from them tends on one hand to make the green

one (image of the black of the vertical diagram) a little whiter, and on the other hand to

counteract the contrast in the purple one by adding to it a little green. Try the experiment with

other glasses than green. (2) Another form of the mirror contrast experiment is as follows.

Place a mirror where the sky or a white surface of some kind will be seen reflected in it. Lay

upon its surface a plate of colored glass, green for example, and hold a little way above it a

narrow strip of black cardboard or a pencil. Two images will be seen: one a vivid green, the

other a complementary purple. The green image belongs to the surface reflection of the colored

glass, as may be proved by observing that when the strip of cardboard touches the surface, the

green image touches it also. The explanation will readily be understood from the accompanying

diagram.

In the diagram, A represents a white surface, C the strip of cardboard,, E the plate of green

glass, F the glass of the mirror. As in Ragona Scinà's experiment, the which surface is reflected

unchanged from the upper surface of the green glass. A good deal of the light, however,

traverses the green glass again, and finally, as a strong green, mixes with the white reflected

from the surface of the green glass, forming, as in Rogona Sainà's experiment, a light green

field. The black strip C is reflected at O, that is to say at O is a place where the white from the

surface A is cut off, and only green from M, by was of S, is present, hence its image appears

green. But C is also reflected at T (or its light is wanting there), so that the white reflected from

R is unmixed with green. By contrast, it appears purple. It is easy, by substituting for C a gray

strip that will send some light through the glass at O and R, to show that contrast can suppress

an actually present objective color. [p. 408]

c. Meyer's experiment. Lay on a large colored held a small piece of gray or even black paper

(e.g., 1 cm. wide by 2 cm. long), and cover the whole with a piece of semi-transparent white

paper. The contrast color will appear on the gray paper. If thin tissue paper is used, more than

one thickness may be needed for the beet result. R. Jung, Heidelberg, sells a book (for

Becker's Florversuche) of alternate leaves of colored and tissue paper with two gray rings

attached, made expressly for Meyer's experiment. Paper mats, woven one way of gray paper

and the other of colored, show this contrast beautifully, as Hering mentions. They may easily be

made from kindergarten materials.

d. Mixed contrasts with the color-mixer. (1) Disks made on the pattern of the cut at the left show

beautiful contracting grays.

The same can be shown also by laying a number of small sheets of tissue paper over one

another in such a way that they partially overlap making a portion where there is but a single

thickness, and next it a portion where there are two thicknesses, and next that again one of

three thicknesses, and so on. When the whole is held up to the light, the contrasts of adjacent

portions are very easily seen. (2) Contrast colors can be shown finely with disks like that in the

cut at the right, in which the shaded portions represent color, the black portions black and the

white, white. A little care is necessary in fixing the proportions of the color to white and black in

the disks for contrast colors, but in general the brightness of the gray should be About that of

the color.

On a cf. Helmholtz, op. cit. F 517-519, 531; G

393-395, 405; G

551-553. Hering Ueber die

Theorie des simultanen Contrastes von Helmholtz; Dir farbingen Schatten, Pflüger's Archiv, XL,

172. V. Bezold, op cit.

On b (1) Helmholtz, op. cit. F 531-532; G.

405-406; G.

557-558. Hering: Ueber die Theorie

des simultanen Contrastes von Helmholtz; Der Speigelcontrastversuch. Pflüger's Archiv, XLI,

1887, 358-367. Wundt op. cit., I, 482. See also the physiologies in general..

On b (2) cf. Dove: Versuche über subjective Complementarfarben, Pogg. Ann., XLV, 1838, 158.

Helmholtz, op. cit. F 532, G.

406, G.

558. V. Bezold, op. cit.

On c. Helmholtz, op. cit. F. 523, 530-531, G.

398, 404-405 G.

547-548. Hering: Ueber die

Theorie des simultanen Contrastes von Helmholtz; Der Contrastversuch von H. Meyer und die

Versuche am Farbenkreisl. Pflüger's Archiv, XLI, 1887, 1-29.

On d. Helmholtz, op. cit. F. 538-543, G.

411-414 G.

544-547. Hering op. cit. on c. V. Bezold,

op. cit. Meyer, Pogg. Ann., XCV, 170. Phil. Mag. Ser. 4 IX, 547.

For quantitative measurements of contrast grays cf. Ebbinghaus, Die Gesetzmässigkeit des

Helligkeitscontrastes Sitzber. d. k. Preus. Akad., Berlin, Sitz. v. 1, Dec., 1887. Lehmann: Ueber

die Anwendung der Methode der mittleren Abstufungen auf den Lichtsinn: Die quantitative

Bestimmung des Lichtcontrastes. Wundt's Philos. Studien, III, 1886, 516-528. [p. 409]

143. Conditions that influence contrast. a. Contrast effects are stronger when the colors are

near together. (1) Lay a bit of white paper on a black surface, e.g., a piece of black velvet, and

notice that the paper is whiter and the velvet blacker near the margin of the paper than

elsewhere, notwithstanding that the eye moves about freely. This has received the name of "

Marginal contrast" (Randcontrast). (2) On a piece of gray paper, the size of a letter-sheet, lay

two strips of colored paper close side by side (e.g., pieces of red and yellow or of green and

blue, 1 cm. wide by 4 cm. long). Below them to the right and left as far apart as the paper will

permit, lay two other strips of the same size and color, red on the red side of the former pair,

yellow on the yellow side. Notice the effect of the difference in distance on the contrasting pairs.

Contrast of this sort is at a minimum when one color entirely surrounds the other.

b. Effect of size. When the area of the inducing color is large and that of the induced color is

small, the contrast is shown chiefly on the latter; when the two areas are of about equal size, as

in a(2) above, the effect is mutual. Try with large and small bits of paper upon a colored field.

c. Borders and lines of demarkation[sic] that separate the contrasting areas tend to lessen the

effect by excluding marginal contrast. Repeat Ex. 142c, using two slips of gray paper 5 mm.

wide by 2 cm. long, substituting a piece of moderately transparent letter paper for the tissue

paper. When the contrast color has been noted, trace the outline of one of the slips with a fine

ink line upon the paper that covers it. Notice that the color nearly or quite vanishes. This

experiment and others like it play an important part in the psychological, as opposed to the

physiological, explanation of simultaneous contrast (see Helmholtz, op. cit. F. 538 f., G.

406 f.,

559 f., but cf. also EX. 144). Such a black border will, however, also make a weak objective

color invisible. A disk like that in the cut accompanying Ex. 142d, when provided with a second

contrast ring, marked off on both its edges with a firm black line, shows a weakening of the

induced color in the bordered ring.

d. Saturation. Contrast effects are generally most striking with little saturated colors. (1)

Compare the effect of increasing decreasing and extinguishing the second non-colored light in

the colored shadow experiments. It is necessary, however, to see to it that reflected light from

the walls and surrounding objects does not complicate the experiment. (2) Compare the

intensity of the contrasts in Meyer's experiment (Ex. 142c) before and after the application of

the tissue paper. Notice also the part played by the white light mixed with the colored light in

the mirror contrast experiments above. Powerful contrasts with the most saturated colors can

be observed, however, when the proper conditions are fulfilled.

On helpful conditions in general cf. Helmholtz, op. cit. F. 513-514, G.

390-391, G.

540-541.

On c. Helmholtz, op. cit. F. 539-542, G.

411-414, G.

546-547.

On d. Helmholtz, op. cit. F. 528-524[sic], G.

399-400.

144. Simultaneous contrast. The effects of simultaneous contrast are often lost in the more

striking ones of successive contrasts, and the first requisite of an experiment on simultaneous

contrast is the exclusion of the successive. This is not difficult in experiments in colored

shadows. a. Place a piece of white paper in such a position that it may be illuminated at once

from the window (if the day be overcast) and from a gas-jet. Set upon it a small block or other

object, about 1½ by 3 inches in size, and either black or white [p. 410] in color. Light the gas

and observe the two shadows, one cast by the light from the window, the other by the gas. The

first will appear yellowish, the second clearly blue.[8] Adjust the distance and position with

reference to the light so that the shadows shall appear about equally dark, and the blue shadow

shall be as sharply bounded as possible, and to that end have the shadow cast by the edge

rather than the flat side of the flame. The color of the yellowish shadow is objective and due to

the yellow of the gas-flame, that of the blue is due to the contrast, but largely as yet to

successive contrast. Put a dot in the centre of the blue shadow, to serve as a fixation-point, and

another on the edge. Fasten a paper tube (preferably blackened inside) so that it can easily be

shifted from one dot to the other. Cut off the gas-light by holding a card between it and the

block; adjust the tube so that the dot in the middle of the shadow may be fixated without any of

the parts of the field outside of the shadow being seen. Wait until all of the blue has

disappeared from the shadow, and then, still looking through the tube, remove the card. The

field remains entirely unchanged and appears, as before, a colorless gray. The former blue

color is thus shown to be subjective and due to contrast with the yellow lighted area in which it

lies. b. Cut off the gas-light again and adjust the tube so that the dot in the edge of the shadow

may be fixated. Taking great precaution not to move the eye, withdraw the card. The part of the

field of the tube filled by the shadow will appear bluish, that of the remainder reddish-yellow.

After a little time of steady fixation, cut off the gas-light once more and observe the instant

reversal of the colors. The shadow now appears in reddish-yellow the rest of the field blues.

The color of the shadow, both before and after the final interposition of the card, is due to

simultaneous contrast, in the first case with the reddish-yellow light, and in the second with its

after-image.

Helmholtz explains all cases of simultaneous contrast as errors of judgment; in the case of the

colored shadow, for example, we mistake the yellow of the gas-lighted field for white and

consequently find the shadow which is really gray to be bluish. In the case of this particular

experiment, Hering and Delabarre seem to have shown this psychological explanation

unnecessary and a physiological one all sufficient, and Hearing has done the same for other

forms of experiments.

Cf. on simultaneous contrast in general. Helmholtz, op. cit. F. 515-547, G.

392-418, G.

542 ff.

Hering. op. cit., under Ex. 142. On colored shadows, cf. Helmholtz, op. cit. F. 517-519, G.

394-

396, G.

551-553.

On Helmholtz's theory, cf. Helmholtz, op. cit. F. 543-538, G.

392, 407-411, G.

543 ff. Hering,

op. cit. under Ex. 142; also, Ueber die Theorie des simultanen Contrastes von Helmholtz: Die

subjective "Trennung des Lichtes in zwei complemtäre Portionen."

Delabarre: Colored Shadows, in AMERICAN JOURNAL OF PSYCHOLOGY, II, 1888-89, pp.

636-643. For quantitative measurements of simultaneous contrast, see Kirschmann: Ueber die

quantitativen Verhältnisse des simultanen Helligkeits und Farben-Contrastes, Wundt's Philos.

Stud., VI, 1890.

145. Simultaneous contrast. Hering's binocular method, a. Use the binocular color-mixer

described above in the note on apparatus. Set a red glass in the right frame, a blue glass in the

left. Look fixedly through the colored glasses of the instrument at the cork ball below, bringing

the eyes close to the glasses and the [p. 411] nose between them. Adjust the side screens till

the white ground below appears in a uniform light violet from the binocular mixture of the red

and blue(cf. Ex. 153). The narrow strip of black paper on the white is seen double, the right

hand image bluish, the left yellowish. b. The possibility of successive contrast is, however, not

yet excluded. That may be accomplished as follows: Lay a sheet of black paper over the whole

of the white field and its black strip; rest the eyes; and finally, when everything is in readiness,

and the eyes again fixed on the ball, swiftly draw away the black paper. The contrast colors are

seen on the instant, before any motions that might introduce successive contrast have been

made.

Hering argues that this experiment is conclusive against the psychological explanation of

simultaneous contrast, unless a separate unconscious judgment is to he made for each eye, for

that which consciously appears is a light violet held, and the contrast color to that should be a

greenish-yellow, and both images of the strip should be alike, whereas, as a matter of fact, the

images appear in different colors, neither of which is the color required.

Hering: Beitrag zur Lehre vom Simultankontrast. Zeitschr. F. Psychol. I. 1890. 18-28). For a

different experiment supporting the same conclusion. see Hering's paper.

146. Influence of judgment in visual perception. While in the previous experiment a

psychological explanation seems sufficient for the facts, psychical action is not excluded, even

by Hering, from a considerable share ill sense perception. In the: following experiments

judgment coöperates in the result. a. Place upon the color-mixer a short-pointed star of white

cardboard, or even a square when in sufficiently rapid rotation, it appears as a white central

circle surrounded by a more or less transparent ring. While it is in rotation bring behind it a

broad strip of black card board of somewhat greater length than the diameter of the star from

point to point. As the edge of the card advances it can be seen not only behind the transparent

ring, but, apparently, also behind the opaque central circle, and the portions of the latter in front

of the black card seem darkened by its presence. The illusion holds, though with a lightening

instead of a darkening effect when a white card is moved behind a black star. The illusion fails

by degrees it the card is kept motionless, but may be observed to a certain extent when the star

is at rest. b. Cover a piece of black cardboard smoothly with tissue paper and notice that it

seems, at first, blacker than it afterwards proves to be on comparison with other grays. c. In

mixing colors by reflection (Ex. 140a). notice the tendency to see one color through the other

instead of seeing the mixture of the two. This tendency may be so strong at first as to interfere,

to a certain extent, with the success of the experiment. Cf. also Ex. 152.

On the difficulty of judging small differences in the color of surface that present other small

unlikenesses, cf. Hering, op. cit. under Ex. 142c.

On a., Sanford. Science, XXI. 1893, 92.

147. An effect exactly the reverse of contrast appears when a figure in black or white is placed

upon a colored ground. The black figure appears to darken the ground and the white to

brighten it. This is a method often used in polychromatic decoration. Observe the effect on the

blue ground in Fig. 4, Plate I. It may be observed occasionally in plaid fabrics, and is shown

very satisfactorily in kindergarten mats woven in checker-board pattern of colored and gray

papers. If a set of grays is used so that the strips may range from a black at one side to a white

at the other, the corresponding shading of the colored paper is striking.

V. Bezold, op. cit. 182-183, and Plate V. [p. 412]

BINOCULAR PHENOMENA OF LIGHT AND COLOR.[9]

148. In general the two eyes coöperate to bring about a single visual result, but the union of the

impressions upon the two retinæ is influenced by a number of circumstances. a. If the stimulus

to one eye is considerably stronger then that to the other, the sensation in the latter is in most

cases totally suppressed. Close one eye and look at a sheet of white paper with the other

letting the open eye move about freely. There is no tendency for the darkened field of the

closed eye to assert itself. b. When, however, the effect of the stimulus in the open eye is

somewhat weakened by steady fixation, such a tendency is to be observed, and the whole of

the field of the open eye, except a small area about the point fixated, may be, from time to time,

suppressed by the dark field of the closed eve. A slight motion will however, instantly restore

the first. Cf. also Ex. 118. c. A field that contains sharply marked objects or contours will

generally triumph over one that does not. Try combining the letters below in such a, may that

the B's are superposed. In these the white field of either eye which corresponds to A or C in the

other eye will generally not triumph over the letters.

On the Binocular Phenomena of Light and Color in general, see Fechner: Ueber einige

Verhältnisse des Binocularen Sehens, Abhandlungen der kgl. sächs. Ges der Wiss. VII. 1860,

pp. 339 584. Helmholtz, op. cit. F 964-999, G.

767-796. Hering: Hermann's Handbuch der

Physiol., III. Th. I, 380 385, 576-577, 591-601; Beiträge zur Physiologie, 308-316. Aubert:

Grundzüge der physiologischen Optik, 499-503, 550-554. Wundt: Physiol. Psychol. 3te. Aufl. II.

177-179, 183-189. Ebbinghaus: Ueber Nachbilder im binocularen Sehen und die binocularen

Farbenerscheinungen überhaupt, Pflüger's Archiv, XLVI. 1890. 498-508. Titchener. Ueber

binoculare Wirkungen monocularer Reize. Wundt's Philos. Studien. VIII. 1892. 231-310.

Chauveau: Several articles in the Comptes rendus, CXIII. 1891, 358, 394, 439, 412.

149. Fechner's paradoxical experiment. Hold close before one eye a dark glass, such as is

used in protecting the eyes, or a piece of ordinary glass moderately smoked over, or even a

black card with a good sized pin-hole in it, allowing the other eye to remain free. It is easy to

see that the binocular field is darkened by the interposition, of the dark glass. If, however, the

eye behind the glass is closed, or the light wholly cut off from it by holding a black card in front

of the glass, the field appears decidedly brighter that is to say, cutting off a portion of the

stimulus received by the total visual apparatus, has caused an increased intensity of sensation.

The experiment fails for very dark and very light glasses. Several explanations have been

given, but that of Aubert, according to which the sensations of the two retinæ blend in a sort of

average result when the difference is not too great, but one wholly suppresses the other when it

is very great, seems to be the most satisfactory and with this Hering also in the main agrees.

150. Rivalry. When the two retinas are stimulated separately with strong light of different colors,

or are confronted with otherwise incongruous fields, i.e., fields that cannot be given a unitary [p.

413]interpretation, there result a peculiar instability and irregular alternation of the colors over

past or the whole of the combined fields of vision. This apparent struggling of the fields is

known as Retinal rivalry. Hold close before one eye a piece of blue glass, before the other a

piece of red glass, and look toward the sky or a brightly lighted uniform wall. The struggle of

colors will at once begin. The same may be observed with a stereoscope when the usual paired

photographs are replaced by colored fields, or even with no apparatus at all, when both eyes

are closed and turned toward a bright sky and one of them covered with the hand. Rivalry has

been explained as due to fluctuations of attention, and some observers find that it can be more

or less controlled by attention (Helmholtz); Fechner discusses the attention theory, and finds it

insufficient. Hering and others regard the changes as of a purely physiological origin. Cf. Ex.

151b.

151. Prevalence and rivalry of contours. By "contours" is here meant lines of separation where

fields of one color border upon fields of another color. a. Combine stereoscopically the two bars

below, and notice that it is the contours that suppress the solid parts of both the black and

white. This figure gives excellent results when colors are substituted for the black and white.

Notice a similar triumph of the contours of the cross over the lines, and of the lines over the

central black of the cross in Fig. XI, or an enlargement of it.

b. Notice the rivalry of the contours in all of these figures. c. Such diagrams as Figs. XI and XII

are suitable for the study of the part played by attention in rivalry. While it is doubtful that mere

attention to one field or the other will cause it to predominate, it yet seems possible by indirect

means to cause it to do so. If attention is given to an examination of the lines and small squares

in Fig. XI, or it one of the series of lines in Fig. XII is counted, they will appear to be somewhat

assisted in their struggle with the cros

[sic] or the other set of lines. d. A printed page has a

decided advantage. [p. 414] Try a diagram in which a printed page is brought into competition

with a field of heavy cross lines. The lines will be found to yield to the print, at least, at the point

at which the reader is looking. Two printed pages, however, become hopelessly mixed, and it is

hard to say how much of the advantage, when a single one is used, is due to its superior power

as a holder of attention, and how much to its excellence as a set of contours. A portion of the

power of contours is probably to be explained by the mutual intensification of both the black

and the white by contrast, but a part is perhaps due to a strong tendency, observable in other

cases also, for the eyes (and attention) to follow lines, and especially outlines.

152. Luster. When one of the rival fields is white and the other colored (especially when one is

white and the other is black) there results, besides the rivalry, a curious illusion of shine or

polish, known as Binocular lustre [sic N.B. Sanford used "Luster" at the start of this section, but

"lustre" thereafter]. a. Examine in the stereoscope a diagram made like the accompanying cut,

and notice the graphite-like shine of the pyramid. The explanation seems to be that polished

surfaces, which at some angles reflect light enough to look white, and at others appear in their

true color have often in previous experience given rise to such differences of sensation in the

two eyes, from which in this instance we infer a polish on the object seen in the diagram. b. A

species of monocular lustre (or transparence) is to be observed when black or white or colors

are combined by means of the reflection color-mixer, especially when the inclination of the plate

is so changed that one color appears to be reflected in the surface of the other or to be seen

through and behind it. The experiment works well when real objects are reflected in the surface

of the glass, the reflecting power of the latter appearing as if transferred to the horizontal

surface on the opposite side.

153. Binocular color-mixing. The result of simultaneous presentation of different colors to the

two eyes is not always rivalry or lustre. If the colors are not too bright and saturated and the

fields are without fleck or spot to give one the predominance, a veritable though somewhat

unsteady mixture of the colors may result. a. Place a red and a blue glass of equal thickness in

the binocular color-mixer, and adjust the side screens till the proper amount of white light is

mixed in with that transmitted from below. The mixture will be seen on the white field below. Try

also with other combinations of glasses. The mixtures obtained in this way are not exactly the

same in appearance as the monocular mixtures studied above. b. The same effect maybe

conveniently obtained with a stereoscope, from which the middle partition has been removed.

[p. 415] Try with equal areas of dull colors of little saturation. Hering recommends two squares

of red and two of blue, set at equal distances in a horizontal line, the two reds on one side, the

two blues on the other. When the middle pair are combined stereoscopically they show a mixed

color while the unmixed colors can be seen for comparison beside them. He also suggests the

use of lenses to prevent sharp focusing of the eyes upon the contours, which interfere with the

mixture. Complementary colors are said to be more difficult to fuse than those standing nearer

in the color scale. Cf. Also Ex. 149. For diagrams that bring in binocular perspective to aid in

mixing the colors and for a specially adapted stereoscope, see Chauveau.[10]

154. Binocular contrast. The side-window experiment. Stand so that the light from the window

falls sidewise into one eye, but not at all into the other. Place in a convenient position for

observation a strip of white paper on a black surface. The paper when looked at with both eyes

appears perfectly colorless. On looking now at a point nearer then the bit of paper (e.g., at the

finger held up before the face), double images of the bit will be seen. The two images will be

different in brightness and slightly tinged with complementary colors. The image belonging to

the eye next the window (which may be recognized by its disappearance when that eye is

closed) will appear tinged with a faint blue or blue-green color, the other with a very faint red or

yellow. The light that enters the eye through the sclerotic is tinged reddish-yellow and makes

the eye less responsive to that color the white of the paper strip therefore appears bluish. It

appears darker partly for a similar reason, and perhaps also, as Fechner suggests, because it

lies in a field which for the eye in question is generally bright. The reddish color of the other

eye's image of the strip is explained as due to contrast with the first, but whether this contrast is

a purely psychical matter or whether it is to be explained by the action of the stimulus in the first

eye upon the second, as there seems some reason to think, is as yet uncertain. Its greater

brightness is probably due to the fresher condition of the eye to which it belongs, and to

contrast with its less brilliant field. The same thing is often to he noticed when reading with the

lamp at one side, or even when one eye has been kept closed for a short time while the other

has been kept open. The double images are in nowise essential; simple alternate winking will

show decided differences in the condition of the two eyes.

155. Binocular after-images. Lay a bit of orange-colored paper on a dark ground, and provide

two white cards. Hold one of the cards close to the left eye, but a little to one side, so as not to

hide the bit of paper. Hold the other eight or ten inches from the right eye in such a way as to

hide the paper. Look at the paper for a few seconds with the left eye, then bring the card before

it. A faint, washy, orange-colored positive after-image will appear on the card before the right

eye. This after-image is supposed to belong to the right eye's half of the visual apparatus,

possibly to the central, i.e., cerebral, part.

Footnotes

[1] For concise statements of these facts, see Wundt, Physiologishe Psychologie. I, 487 (cited

by Ladd. Phys. Psych., 338), also p. 501, and Christine Ladd Franklin. Zeit. für Psych. IV,

19892, 212.

[2] The second edition of Helmholtz's great work is as yet incomplete. The latest complete

edition is the French translation. Optique Physiologique, Paris, 1867. To facilitate reference

when pages are cited, the numbers are given preceded by G

for the second German edition,

and by G

for the first German edition. and by F. for the French translation. Occasional errors in

the pages for G

may hare crept in, for that edition was not at hand and the pages for it have

been taken from the double paging in G

and F. The error can hardly amount, however. to

more than a page one way or the other.

[3] It is difficult to give the tints accurately in words. The experimenter should consult the

colored charts given in the books of Jeffries mentioned in the bibliography.

[4] The formula (or the amount of black, assuming that the radial line is absolutely black, and

taking some arbitrary point of the line. e.g., the middle point, for the calculation. is of course

b/2pr, where b is the breadth of the radial line and r the distance of the chosen point from the

centre of the disk.

[5] Since the black of the disk is really a very dark gray, this is not absolutely pure experiment,

but is sufficiently exact for the purpose.

[6] This law of course has reference to the mixture of colored lights and not to mixtures of

pigments upon the palette.

[7] A small portion is also reflected from the lower surface of the glass, so contributes a small

amount of green.

[8] This setting of the experiment succeeds best when the daylight is weak, as for example, just

before the lights are usually lighted in the evening. If the experiment is to he made in broad

day, the light must be reduced by curtains or otherwise; if at night there must be two lights. one

corresponding to the window and one to the gas, and the latter must shine through a pane of

colored glass. If yellow glass is used the colors will be the same as those in this experiment.

[9] The experiments that follow can all be made with the stereoscope. but practice will enable

the experimenter to combine the diagrams with free eyes. either by crossing the lines of sight

(fixating a point nearer than the diagram), or by making them parallel or nearly so (fixating a

point beyond the diagram). This skill the experimenter should try to acquire. In these

experiments it is important that the eyes should be of approximately equal power and if the

poorer eye cannot be helped with lenses, the vision of the other must be some what reduced by

the interposition of a sufficient number of plates of ordinary glass.

[10] Comptes rendus. CXIII, 1891, p. 442.

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