scope, and took into consideration the principle of reflection; and as he found that rays of all colours were reflected regularly, so that the angle of reflection was equal to the angle of incidence, he concluded that, upon this principle, optical instruments might be brought to any degree of perfection imaginable, provided a reflecting substance could be found which could polish as finely as glass, and reflect as much light as glass transmits, and provided a method of communicating to it a parabolic figure could be obtained. These difficulties, however, appeared to him very great, and he even thought them insuperable when he considered that, as any irregularity in a reflecting surface makes the rays deviate five or six times more from their true path than similar irregularities in a refracting surface, a much greater degree of nicety would be required in figuring reflecting.specula than refracting lenses.

Such was the progress of Newton's optical discoveries, when he was forced to quit Cambridge in 1666 by the plague which then desolated England, and more than two years elapsed before he proceeded any farther. In 1668 he resumed the inquiry, and having thought of a delicate method of polishing, proper for metals, by which, as he conceived, "the figure would be corrected to the last," he began to put this method to the test of experiment. At this time he was acquainted with the proposal of Mr. James Gregory, contained in his Optica Promota, to construct a reflecting telescope with two concave specula, the largest of which had a hole in the middle of the larger speculum, to transmit the light to an eye-glass;* but he conceived that it would be

*M. Biot, in his Life of Newton, has stated that Newton was preceded in the invention of the reflecting telescope by Gregory, but proba bly without knowing it. It is quite certain, however, that Newton was acquainted with Gregory's invention, as appears from the following avowal of it. "When I first applied myself to try the effects of reflection, Mr. Gregory's Optica Promota (printed in the year 1663) having fallen into my hands, where there is an instrument described with a hole

an improvement on this instrument to place the eyeglass at the side of the tube, and to reflect the rays to it by an oval plane speculum. One of these instruments he actually executed with his own hands; and he gave an account of it in a letter to a friend, dated February 23d, 1668-9, a letter which is also remarkable for containing the first allusion to his discoveries respecting colours. Previous to this he was in correspondence on the subject with Mr. Ent, afterward Sir George Ent, one of the original council of the Royal Society, an eminent medical writer of his day, and President of the College of Physicians. In a letter to Mr. Ent he had promised an account of his telescope to their mutual friend, and the letter to which we now allude contained the fulfilment of that promise. The telescope was six inches long. It bore an aperture in the large speculum something more than an inch, and as the eyeglass was a plano-convex lens, whose focal length was one-sixth or one-seventh of an inch, it magnified about forty times, which, as Newton remarks, was more than any six-foot tube (meaning refracting telescopes) could do with distinctness. On account of the badness of the materials, however, and the want of a good polish, it represented objects less distinct than a six-feet tube, though he still thought it would be equal to a three or four feet tube directed to common objects. He had seen through it Jupiter distinctly with his four satellites, and also the horns or moon-like phases of Venus, though this last phenomenon required some niceness in adjusting the instrument.

Although Newton considered this little instru

in the midst of the object-glass, to transmit the light to an eye-glass placed behind it, I had thence an occasion of considering that sort of construction, and found their disadvantages so great, that I saw it necessary before I attempted any thing in the practice to alter the design of them and place the eye-glass at the side of the tube rather than at the middle. -Letter to Oldenburg, May 4th, 1672

ment as in itself contemptible, yet he regarded it as an "epitome of what might be done;" and he expressed his conviction that a six-feet telescope might be made after this method, which would perform as well as a sixty or a hundred feet telescope made in the common way; and that if a common refracting telescope could be made of the "purest glass exquisitely polished, with the best figure that any geometrician (Descartes, &c.) hath or can design,” it would scarcely perform better than a common telescope. This, he adds, may seem a paradoxical assertion, yet he continues, “it is the necessary consequence of some experiments which I have made concerning the nature of light."

The telescope now described possesses a very peculiar interest, as being the first reflecting one which was ever executed and directed to the heavens. James Gregory, indeed, had attempted, in 1664 or 1665, to construct his instrument. He employed Messrs. Rives and Cox, who were celebrated glassgrinders of that time, to execute a concave speculum of six feet radius, and likewise a small one; but as they had failed in polishing the large one, and as Mr. Gregory was on the eve of going abroad, he troubled himself no farther about the experiment, and the tube of the telescope was never made. Some time afterward, indeed, he "made some trials both with a little concave and convex speculum," but, "possessed with the fancy of the defective figure, he would not be at the pains to fix every thing in its due distance."

Such were the earliest attempts to construct the reflecting telescope, that noble instrument which has since effected such splendid discoveries in astronomy. Looking back from the present advanced state of practical science, how great is the contrast between the loose specula of Gregory and the fine Gregorian telescopes of Hadley, Short, and Veitch,

between the humble six-inch tube of Newton and the gigantic instruments of Herschel and Ramage.

The success of this first experiment inspired Newton with fresh zeal, and though his mind was now occupied with his optical discoveries, with the elements of his method of fluxions, and with the ex.. panding germ of his theory of universal gravitation, yet with all the ardour of youth he applied himself to the laborious operation of executing another reflecting telescope with his own hands. This instrument, which was better than the first, though it lay by him several years, excited some interest at Cambridge; and Sir Isaac himself informs us, that one of the fellows of Trinity College had completed a telescope of the same kind, which he considered as somewhat superior to his own. The existence of these telescopes having become known to the Royal Society, Newton was requested to send his instrument for examination to that learned body. He accordingly transmitted it to Mr. Oldenburg in December, 1671, and from this epoch his name began to acquire that celebrity by which it has been so peculiarly distinguished.

On the 11th of January, 1672, it was announced to the Royal Society that his reflecting telescope had been shown to the king, and had been examined by the president, Sir Robert Moray, Sir Paul Neale, Sir Christopher Wren, and Mr. Hook. These gentlemen entertained so high an opinion of it, that, in order to secure the honour of the contrivance to its author, they advised the inventor to send a drawing and description of it to Mr. Huygens at Paris. Mr. Oldenburg accordingly drew up a description of it in Latin, which, after being corrected by Mr. Newton, was transmitted to that eminent philosopher. This telescope, of which the annexed is an accurate drawing, is carefully preserved in the library of the Royal Society of London, with the following inscription:

"Invented by Sir Isaac Newton and made with his own hands 1671."