Page:The American Cyclopædia (1879) Volume XV.djvu/657

 TELESCOPE 627 01

e performance of the instrument. Not long afterward Gauss remarked that it was possi- ble, theoretically, to construct an object glass which would unite all the rays of any two lors as well as the mean rays at the centre " at a given distance therefrom into one and e same point. Both lenses should be con- cavo-convex. With a proportion of aperture to focal length of -fa he obtained an almost perfect union of rays. The unusually deep urvatures of the lenses seem to have occa- oned some scruples on the part of opticians, d this construction remained almost for- tten for 40 years, until Steinheil found and conquered the practical difficulty, and in 1860 arrived at complete success in the manufacture of the Gaussian object glasses. The proper >nstruction of eye pieces was also a mat- r of some consideration. Besides the Huy- genian form, which is only applicable for viewing objects, Ramsden in 1783 introduced another, which is still used in micrometer ob- servations. It consists of two plano-convex lenses, of equal focus, with their convex sur- faces toward each other, and separated by a dis- tance of two thirds of the common focal length. By this arrangement, to which he was guided by a remark of Newton, the essential condi- tion of a "flat field" is gained, and the aberra- tions, chromatic and spherical, are so much re- duced as to be practically insensible. For ter- restrial observations, the elder Dollond sought to reduce aberrations and enlarge the field of view by increasing the number of lenses, and, after improving the four-glass eye pieces al- ready in use, obtained by adding a fifth lens a combination which very satisfactorily effected both the desired objects. Joseph Fraunhofer studied the theory of light and the laws to which it is subject in transmission through various media, and solved the difficulty of procuring disks of homogeneous flint glass. The process by which his glass was manufac- tured is kept a secret, but it is generally un- derstood that the disks themselves are obtained by selecting and melting together the most faultless specimens from larger masses of the best glass, whose constituent parts however are not known. Having now the glass, he well knew how to combine curvatures to suit its peculiar properties, and the results are to be found all over Europe. He completed in 1824 the splendid telescope for the observatory at Dorpat. The object glass of this instrument, double and not triple as sometimes stated, has a clear aperture of 9 -6 in., and a focal length of 170'5 in. Its optical performance is of the highest character. It gave to the stellar im- ages a perfect sharpness of definition, which enabled it not only to resolve the closest known double stars, but also to discover as double or multiple others that had passed unchallenged before the exquisite 20-foot reflectors and the practised eye of the younger Herschel. Fraun- hofer's style of "mounting" the telescope re- mains to this day essentially unimproved. The manufacture of optical glass has received much attention in England. In 1824 a com- mittee was appointed by the royal society to take into consideration the theory and to ex- periment upon the manufacture of such glass. The chief labor devolved upon three members, G. Dollond, Faraday, and Herschel. The first results were reported to the society in 1829. The efforts of this committee were directed to the manufacture of very heavy glass, and they obtained disks of 7 in., which seemed, so far as tried, to answer all the requirements of the telescope. Dr. Kitchie also devoted much at- tention for several years to the same subject, and with considerable success, but was pre- vented by premature death from publication of any of his processes. Judging by the appear- ance^ of Ritchie's glass, Mr. Simms inferred that it had been fused in moulds and there sub- jected to pressure. The largest disk had 7f in. diameter, and was ground for use by Simms himself. It was found to be "an excellent glass, but not altogether faultless." The idea occurred to some that the desired achroma- ticity might be obtained by separating the lenses and placing the flint at some distance down the tube in the narrowing cone of rays. In 1828 Alexander Rogers proposed to introduce in combination with the crown lens a smaller compound lens of plate and flint glass, in which the refraction is en- tirely destroyed, and the outstanding disper- sion left available for the desired correction of that of the outer lens. The investigation of the requisite curvatures of this compound lens was found to present no peculiar difficulty ; and moreover the final perfection of the com- pensating action could be accomplished by proper adjustment of the relative positions of the lenses, so that less rigorous accuracy is requisite in their mechanical formation. Rogers found it probable that a telescope of 18 ft. focal length, with a crown lens of 12 in. aperture, could be made achromatic with a flint lens only 4 in. in diameter; and four years later this construction was introduced into use by Plossl at Vienna with much success. It received the name of "dialytic" or separated telescope. One of these telescopes, in the possession of Schumacher, having an aperture of 2 in. and focal length of 2 ft., was de- scribed by him as of extraordinary excellence of defining power. Struve compared a dialytic telescope of 3J in. aperture, bearing a magni- fying power of 135, with a Fraunhofer tel- escope of half an inch greater aperture and a power of 210, and was scarcely able to per- ceive any superiority in the latter. Telescopes with lenses of rock crystal and glass were advertised to be made in Paris by Cauchoix in 1831, and some few came into favorable notice; but the difficulty of obtaining the ma- terials in proper shape and size will be a per- manent obstacle to their general manufacture. It had long been observed that, even in the best telescopes, there were residual colors hav-