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so that if the rays of light were globular bodies, they might acquire a circulating motion by their oblique passage out of one medium into another, and thus move like the tennis-ball in a curve line. Notwithstanding, however, “this plausible ground of suspicion,” he could discover no such curvature in their direction, and, what was enough for his purpose, he observed that the difference between the length MN of the image, and the diameter of the hole H, was proportional to their distance HM, which could not have happened had the rays moved in curvilineal paths.
These different hypotheses, or suspicions, as Newton calls them, being thus gradually removed, he was at length led to an experiment which determined beyond a doubt the true cause of the elongation of the coloured image. Having taken a board with a small hole in it, he placed it behind the face BC of the prism, and close to it, so that he could transmit through the hole any one of the colours in MN, and keep back all the rest. When the hole, for example, was near C, no other light but the red fell upon the wall at N. He then placed behind N another board with a hole in it, and behind this board he placed another prism, so as to receive the red light at N, which passed through this hole in the second board. He then turned round the first prism ABC so as to make all the colours pass in succession through these two holes, and he marked their places on the wall. From the variation of these places, he saw that the red rays at N were less refracted by the second prism than the orange rays, the orange less than the yellow, and so on, the violet being more refracted than all the rest. "
Hence he drew the grand conclusion, that light was not homogeneous, but consisted of rays, some of which were more refrangible than others.
As soon as this important truth was established, Sir Isaac saw that a lens which refracts light exactly
like a prism must also refract the differently coloured rays with different degrees of force, bringing the violet rays to a focus nearer the glass than the red rays. This is shown in fig. 2, where LL is a convex lens, and S, L, SL rays of the sun falling upon it
in parallel directions. The violet rays existing in the white light SL being more refrangible than the rest, will be more refracted or bent, and will meet at V, forming there a violet image of the sun. In like manner the yellow rays will form an image of the sun at Y, and so on, the red rays, which are the least refrangible, being brought to a focus at R, and there forming a red image of the sun.
Hence, if we suppose LL to be the object-glass of a telescope directed to the sun, and MM an eye-glass through which the eye at E sees magnified the image or picture of the sun formed by LL, it cannot see distinctly all the different images between R and V. If it is adjusted so as to see distinctly the yellow image at Y, as it is in the figure, it will not see distinctly either the red or violet images, nor indeed any of them but the yellow one. There will consequently be a distinct yellow image, with indistinct images of all the other colours, producing great confusion and indistinctness of vision. As soon as Sir Isaac perceived this result of his discovery, he aban. doned his attempts to improve the refracting tele. 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, 6 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 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.
* M. Biot, in his Life of Newton, has stated that Newton was preceded in the invention of the reflecting telescope by Gregory, but proba. Oly 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
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 oigantic instruments of Herschel and Ramage.