When, however, Newton had discovered the unequal refrangibility of light, and had ascertained that the aberration produced by this cause about the focus of a lens was many hundred times greater than that which was caused by the spherical form of the glass, he gave up the hope of being able to construct refracting telescopes which should be free from this defect, and applied himself to the formation of specula for those of the catoptrio kind : the image formed by reflection from a mirror being free from what is called the chromatic and consequently incomparably more distinct than one which is formed by the refraction of light in a lens of any transparent medium.
In the beginning of 1669, Newton having obtained a composition of metals which appeared likely to servo for a mirror, began with his own hands to grind its surface to a spherical form ; and early in the year 1672 he completed two telescopes : of the construction and performance of these instruments he sent to the Royal Society an account which was read in the January of that year. The radius of the concave metal in one of them was 13 inches, and the telescope magnified about 33 times. The rays, before forming an image in the focus of the speculum, were intercepted by a glass prism, or a plane mirror, and the image formed after this second reflection was viewed by a convex eye-glass which was fixed for the purpose in the side of the tube. In the telescope proposed by Gregory, the rays in each pencil of light, after crossing at the focus of the great speculum, were to fall upon the surface of a small ;concave mirror ; and by this being again reflected, they were to form a second image near the anterior surface of the first speculum : through a perforation in the latter the image was to be viewed ; a convex lens being interposed between the image and the eye of the observer. This has been always called the Gregorian telescope; and in 1672, the year in which Newton completed, his reflecting telescopes, M. Cassegrain, in France, proposed one which differed from that of Gregory only in the rays reflected from the great speculum being Intercepted by a small convex mirror; from this the rays of each pencil were again reflected, and they were made to form an image near the anterior surface of the great speculum; this image was to be viewed through a convex lens behind an aperture in the latter specu lum, as in the telescope of Gregory. It does not appear that M. Cassegrain constructed such a telescope, but it may be observed that the image formed. after reflection from the convex speculum would be more free from the aberration caused by the surfaces of the mirrors, and would also be rather greater, than that which is obtained from the concave speculum of Gregory, or the plane one which was used by Newton.
The first reflecting telescope, in which the great speculum was perforated so that objects could be viewed by looking directly at them, was executed' by Dr. Hooke, and produced before the Royal Society in
February, 1674. ,But the difficulty of obtaining metal proper for the purpose, and of giving it a perfectly spherical form, for a long time prevented reflecting telescopes from attaining the desired degree of perfection. In 1718 Mr. Hadley succeeded in executing two teles copes, each about five feet long, which were considered good; and he gave, in the Philosophical Transactions' (1723), a description of the methods employed in their construction. By his advice Dr. Bradley, who was then professor of astronomy at Oxford, in conjunction with Mr. Molyneux at Kew, applied themselves to the construction of these instruments : having executed one which was satisfactory, they in 1738 instructed Scarlet and Hearne, two London opticians, in the processes which they used, and these artists presently succeeded in making good reflecting telescopes for general sale. Mr. James Short of Edinburgh, also soon afterwards distinguished himself by hie skill in forming such telescopes : he attempted at first to make the principal speculum of glass, but finding that this 'material had not sufficient steadiness to preserve the form of its surface, he devoted himself to the improvement of metallic specula, and succeeded in giving them, it is supposed, a correct parabolic figure, by which means his telescopes admitted of larger apertures than any that had before been made.
The processes adopted by Mr. Mudge in grinding and polishing the mirrors for reflecting telescopes, and in giving them the parabolic figure, may be seen in the Philosophical Transactions' for 1777. See also SPECTLUSI.
But the reflecting telescope was carried to a high degree of excellence by Dr. (afterwards Sir William) Herschel. This distinguished astro nomer, while residing at Bath, employed his leisure hours in grinding and polishing specula, with which he formed telescopes, both of the Newtonian and Gregorian kinds; and about the end of 1783, that is subsequently to the discovery of the planet which is sometimes called by his name, being aided by the liberality of the king (George he began the formation of a apeculum four feet in diameter and forty feet in focal length : the telescope to which it appertains is of the Newtonian kind, the observer being placed in a seat near the open end of the tube, and viewing the imago through a system of eye-glasses. With this telescope, which was completed in 1789, objects are magnified about' 6500 times; and on the night after it was finished, Dr. Herschel dis covered the sixth satellite of Saturn. The attempts that have been made to form a reflecting telescope possessing a higher degree of perfection than that of Herschel will be noticed presently.