62G TELESCOPE so that the image of the object was formed at I a, near the eye lens O, which magnified it into I' a'. It is commonly said that the sixth satellite of Saturn was discovered with it ; but this is a mistake, the satellite having been in reality detected with one of Herschel's 18 -inch reflectors. After the lapse of 50 years, during the latter portion of which the telescope had lain unused, it was dismounted by Sir John Her- schel at the end of 1839, and on New Year's eve his family assembled within the tube and sang its requiem. It now rests horizontally upon three stone pillars, a monument to the memory of its constructor. Newton evidently conceived that the prismatic rays of light, once separated, could not be recomposed into white light ex- cept by the same refraction that had separated them, and that therefore the removal of these colors from a telescopic image was impossible. The weight of Newton's authority was suffi- cient for a time to repress further investiga- tions in this direction ; and it was not till 1729 that an Englishman named Hall, guided, it is said, by a study of the mechanism of the eye, was led to a plan of combining lenses so as to produce an image free from colors. Telescopes were made according to his directions, and were said to perform well ; but the secret of their construction died with him, and no public account of the facts was given until called forth by later occurrences. In 1747 Euler, referring to the construction of the human eye, declared that a combination of lenses of different media was possible which should give a colorless im- age, and investigated analytically the curvatures for a lens compounded of glass and water. His result was questioned by the man from whom opposition might have been least expected, John Dollond, who, relying too implicitly upon Newton's dictum, was contending against his own future fame. But he was soon led to con- sider the subject more attentively by the remark of a Swedish mathematician, that there were certainly some cases to which Newton's rules did not apply. He undertook experiments, at first with prisms of glass and water, and soon found that when the prisms were so combined that the rays passed through without refrac- tion, they were tinged with the colors ; next, arranging the prisms so that the rays appeared without colors, he found them displaced by refraction. He arrived at the same results by using prisms of crown and flint glass. From prisms to lenses the transition was easy, and his triumph was finally completed, when, hav- ing combined a convex lens of crown glass with a suitable concave of flint, he was able to correct the colors and leave sufficient refrac- tion outstanding to produce a telescopic image. Euler still believed all kinds of glass alike in their optical properties, and that it was only some happy combination of curvatures at which Dollond had arrived; but his doubts soon gave way before experience, and the masterly pow- ers of his analysis were brought to bear suc- cessfully upon the problem of the compound object glasses. The subject attracted univer- sal attention, and mathematicians everywhere contributed toward perfecting by theory the requisite conditions of curvature of the lenses. The new telescopes were called achromatic, or free from color, and henceforth the " dispersive power " of any medium, by virtue of which the differently colored rays are differently refract- ed (that is, are dispersed from each other), was recognized as independent of the refractive power," by virtue of which the whole pencil is diverted from its original source. Attempting, in 1758, to make double object glasses of short focal distance to be used with a concave eye lens, Dollond found difficulties in the manage- ment of the spherical aberration, and there- upon the idea occurred to him of dividing this aberration by having two lenses of crown glass and including the flint lens between them; an arrangement which accomplished the purpose in view, but did not succeed with convex eye pieces also. His son Peter resumed these experiments, and presented to the royal society of London a triple object glass of 3^ ft. focal length and 3f in. aperture, with which the telescopic image was pro- nounced by Short, an excellent judge, to be " distinct, bright, and free from colors." A beautiful suggestion was made by Wollaston of a means of testing and correcting the con- centric adjustment of lenses. By removing the eye glass of a telescope and viewing any bright object, as a lighted candle, through the object glass, there may be observed at the same time with the refracted image a series of fainter images formed by the second reflec- tions from the different surfaces. It is evident, then, that if the glasses be truly centred, these images will all be in the same straight line ; or if there be any error of position of either lens, it will be decidedly manifested, and by proper adjusting screws may be corrected according- ly. Among the many mathematical solutions of the new problem of the object glasses were the precepts given by Klugel, in his "Diop- trics," viz. : 1, that the radii of curvature of the first, or crown lens, should be such that the angles of the incident ray with the nor- mal would be equal at both surfaces, which would give for crown glass a ratio of nearly 1 to 3 ; 2, the radius of the third surface, the first of the flint lens, should be such that the rays of mean refrangibility passing through both the centre and edge of the lens would unite as nearly as possible in the same part of the axis, so that the spherical aberration would be sensibly destroyed; and 3, having determined the outstanding dispersion for the red and violet rays, the fourth surface should be made such as to unite these rays as near- ly as possible in the same point with the rest. Early in 1816 Bohnenberger, commenting upon these precepts, showed that, by changing the ratio of the first two surfaces from -J- to f , the proportion of aperture to focal length could be materially increased without prejudice to
