An Accompt of a New Catadioptrical Telescope invented by Mr.

Newton

Isaac Newton

SOURCE: 'An Accompt of a New Catadioptrical Telescope invented by Mr. Newton': Philosophical

Transactions of the Royal Society No.81, published 25 March 1672

AUTHOR: Isaac Newton and Christian Huygens

HAND: Printed

REVISION HISTORY

Tagged transcription by Linda Cross January 2003.

thereupon presented the Curious World with an Essay of what may be performed by such

Telescopes; by which it is found, that Telescopical Tubes may be considerably shortned without

prejudice to their magnifying effect.

This new instrument is composed of two Metallin Speculum's, the one Concave, (instead of an

Object-glass) the other Plain; and also of a small plano-convex Eye-Glass.

By Figure I. of Tab. I. the structure of it may be easily imagined; viz. That the Tube of this Telescope

is open at the end which respects the object; that the other end is close, where the said Concave is

laid, and that near the open end there is a flat oval speculum, made as small as may be, the less to

obstruct the entrance of the rays of Light, and inclined towards the upper part of the Tube, where is

a little hole furnish't with the said Eye-glass. So that the rays coming from the object, do first fall on

A B is the Concave speculum, of which the radius or semidiameter is 122/3 or 13 inches.

C D another metalline speculum, whose surface is flat, and the circumference oval.

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G D an Iron wire, holding a ring of brass, in which the speculum C D is fixed.

F, a small Eye-glass flat above, and convex below, of the twelfth part of an inch radius, if not less;

forasmuch as the metal collects the Sun's rays at 62/3 inches distance, and the Eye-glass at less

than 1/6 of an inch distance from its vertex: Besides that the Author (as he informs us) knew their

dimensions by the tools to which they were ground, and particularly measuring the diameter of the

hemi-spherical Concave, in which the Eye-glass was wrought, found it the sixth part of an inch.

G G G, the fore part of the Tube fastn'd to a brass-ring H I, to keep it immoveable.

P Q K L, the hind-part of the Tube, fastn'd to another brass-ring P Q.

O, an Iron hook fastn'd to the Ring P Q, and furnish't with a screw N, thereby to advance or draw

back the hind-part of the Tube, and so by that means to put the specula in their due distance.

M Q G I a crooked Iron sustaining the Tube, and fastned by the nail R to the Ball and Socket S,

distances of the focus E from the vertex's of the little Eye-glass and the Concave speculum, that is,

E F, 1/6 of an inch, and E T V, 61/3 inches; and the ratio will be found as 1 to 38; whereby it

appears, that the Objects will be magnified about 38 times. To which proportion is very

consentaneous, an Observation of the Crown on the weather-cock, about 300 feet distant. For the

scheme X fig.2. represents it bigger by 21/2 times in diameter, when seen <(4006)> through this,

than through an ordinary Telescope of about 2 foot long. And so supposing this ordinary one to

magnifie 13 or 14 times, as by the description it should, this new one by the Experiment must

magnifie near as much as hath been assigned.

Thus far as to the struture of this Telescope. Concerning the Metalline matter, fit for these reflecting

Speculums, the Inventor hath also considered the same, as may be seen by two of his Letters,

as it ought to do. Thus Tin-glass mixt with ordinary Bell-metall makes it more white and apt to reflect

a greater quantity of light; but withall its fumes, raised in the fusion, like so many aerial bubles, fill

the metall full of those Microscopical pores. But white Arsenick both blanches the Metall and leaves

it solid without any such pores, especially if the fusion hath not been too violent. What the Stellate

Regulus of Mars (which I have sometimes used) or other such like substance will do, deserves

particular examination.

To this he adds this further intimation, that Putty or other such like powder, with which 'tis polished,

by the sharp angles of its particles fretteth the metall, if it be not very fine, and fills it full of such

small holes, as he speaketh of. Wherefore care must be taken of that, before judgment be given,

whether the metall be throughout the body of it porous or not.

the pernicious fumes. After this, he put in Tin, and again so soon as that was melted (which was

very suddenly) he stirred them well together, and immediately powred them off.

He saith, he knows not, whether by letting them stand longer on the fire after the Tin was melted, a

higher degree of fusion would have made the metall porous; but he thought that way he proceeded

to be safest.

He adds, that in that metall, which he sent to London, there was no Arsenick, but a small proportion

of Silver; as he remembers, one shilling in three ounces of metall. But he thought withall, that the

Silver did as much harm in making the metall soft, and so less fit to be polish't, as good in rendring it

white and luminous.

At another time he mixed Arsenick one ounce, Copper six ounces, and Tin two ounces: And this an

Acquaintance of his hath, as he intimates, polish't better, than he did the other.

As to the objection, that with this kind of Perspectives, objects are difficultly found, he answers in

that magnifie much; and that after a little use the inconvenience will grow less, seeing that himself

by which the Tube is susteined; or by an ordinary perspective glass fastn'd to the same frame with

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So far the Inventors Letters touching this Instrument: of which having communicated the description

to Monsieur Christian Hugens de Zulichem, we received from him an Answer to this effect, in his

Letter of Febr. 13. 1672. st.n.

I see by the Description, you have sent me of Mr. Newtons admirable Telescope, that he hath well

considered the advantage, which a Concave speculum hath above Convex glasses in collecting the

parallel rays, which certainly according to the calculation, I have made thereof, is very great. Hence

of them by the obscurity of their matter.

Mean time, the main business will be, to find a matter for this speculum that will bear so good and

even a polish as Glasses, and a way of giving this polish without vitiating the spherical figure.

Hitherto I have found no Specula, that had near so good a polish as Glass; and if M. Newton hath

not already found a way to make it better, than ordinarily I apprehend, his Telescopes will not so

inconvenience, and I despair not of finding one. I believe, that M. Newton hath not been without

considering the advantage, which a Parabolical speculum would have above a Spherical one in this

construction; but that he despairs, as well as do I, of working other surfaces than spherical ones

with due exactness; though else it be more easie to make a Parabolical than Elliptical or

Inventor of this new Telescope, from Cambridge, viz.

repect or other indisposed, or that the metals are tarnished. And by your Letter of March 16. I am

fully confimed in that opinion. For, whilest I had it, it represented the Moon in some parts of it as

distinctly, as other Telescopes usually do which magnifie as much as that. Yet I very well know, that

the Instrument hath its imperfections both in the composition of the metall, and in its being badly

cast, as you may perceive by a scabrous place near the middle of the metall of it on the polished

side, and also in the figure of that metall near that scabrous place. And in all those respects that

instrument is capable of further improvement.

distances. But if for such distances, suppose 500 feet, it magnifie at that rate, by the rules of

Opticks it must for the greatest distance imaginable magnifie more than 373/4 to 1; which is so

considerable a diminishing, that it may be even then as 38 to 1.

a six foot Telescope, and found it not only to magnifie more, but also more distinctly. And to day I

than an inch and a third part of the Object-metall. This may be of some use to those that shall

N. B. The Reader may expect in the next Month another Letter, which came somewhat too late to

be here inserted; containing a Table, calculated by the same Mr. Newton, about the several

Apertures and Charges answering the several Lengths of these Telescopes.