TELESCOPE 629 makers it would have passed as optically per- fect. Nevertheless four months more were spent on it, and it was not till October that it was reported finished. . . . The influence of temperature on its figure was now quite per- ceptible. In the evening, while temperature was falling, the defect of the spherical aberra- tion was one way, but after it became station- ry the defect was slightly in the opposite rection." The telescope was mounted at Washington in 1874, and though as yet it has achieved no noteworthy discovery, the ease with which it has gone through the work which had been usually regarded as closely ig the powers of the largest telescopes lows what it is capable of. In England, the ittention of the mechanical astronomers, if we my so call them, has been of late years more specially occupied with the construction of reflecting telescopes, and preeminent in lis department was Lord Eosse, who about 1844 completed a telescope which has a clear srture of 6 ft. and a focal length of 63 ft. lis enormous instrument has two specula, one ibout 3 and the other about 4 tons in weight, 'it first each rested upon a system of 27 plat- forms most ingeniously arranged to distribute "leir support of this enormous weight in such manner as to produce equal pressure in every sition of the instrument. A strong pressure the hand at the back of a speculum 4 tons weight and nearly 6 in. thick produces flex- sufficient to distort the image of a star. tt a later period 27 triangles, each with a ball at each angle, were substituted for the plat- forms, so that now the speculum rolls freely )n 81 balls. The tube of the telescope is sup- >rted upon a massive universal joint of cast >n resting upon a pier of stonework, and it so counterpoised by a chain suspension ap- lied n,t the centre of gravity that it can be loved with great facility, a quick motion being jiven by a windlass below, and a controlling low motion in either direction by the hand of le observer above. Various micrometers have tried with this instrument, but the com- lon filar micrometer with coarse threads an- swers best ; and such is the quantity of light Elected by the immense reflecting surface below, that the threads in the micrometer are always distinctly visible without artificial il- lumination even in the darkest night. The general processes of casting, grinding, and fig- uring these large specula are described in the article SPECULUM. Several other large reflec- tors have been constructed by Lassell, De la Kue, and Nasmyth ; and the first of these transported to Malta a Newtonian telescope 4 ft. in diameter. De la Rue successfully applied his large telescopes to celestial photography, in which he has made many important improve- ments. The manufacture of reflecting tele- scopes with glass specula received a new im- pulse from the discovery by Liebig of a process of coating glass with an infinitesimal film of pure metallic silver. From the first days of reflectors, as early as Newton, we find a prop- osition to _ substitute a silvered lens for the metallic mirror of his telescope, on account of the greater perfection with which glass could be wrought, and the greater durability of the polished surface. In 1740 Caleb Smith showed how, with glass mirrors silvered upon the pos- terior surface, the rays of different refrangi- bility, after twice passing through the glass, and thus becoming separated, might be united again by the action of a small concave lens placed not far from the focus of the mirror. The elder Herschel sometimes used glass reflec- tors for his smaller telescopes. In 1822 Airy proposed a combination of two silvered lenses in the Gregorian or Cassegrainian form, and showed how, by proper mutual adjustment of the two, a perfect achromatism might be ob- tained. In 1838, and again in 1841, Barfuss of Weimar found that, of the various forms of reflectors, the Cassegrainian was best adapted for glass mirrors. He demonstrated that in this form both chromatic and spherical aberra- tion may be sensibly corrected in a telescope of 20 in. focus with full 5 in. aperture, and that such a telescope would bear even a power of 600. But by Liebig's discovery a still better field has been opened. His process consists in precipitating the silver upon the glass surface from an alkaline solution prepared by addition of caustic soda to the ammonio-nitrate. After immersing the glass for about three quarters of an hour, an extremely thin and regular film is obtained, which has a slight bronzy hue by reflected light, and will transmit a deep blue light when interposed between the sun and the eye. This film is said to be harder than ordinary silver, and, by friction with soft leather and perhaps a little dry rouge, is sus- ceptible of receiving the most brilliant polish externally, while it answers perfectly in figure to that of the glass beneath. Foucault has also made use of a similar process (see SPECU- LUM), and succeeded in constructing telescopes of considerable dimensions. One was made by him of 13 in. aperture and only 88 in. focus, with which, under a magnifying power of 600, he could separate the components of the small companion of y Andromedse. Steinheil, in- vestigating the relative reflecting power of a speculum coated by this new process, as com- pared with others and with the transmitting power of some object glasses, found that, under an angle of reflection of 45, the amount of brightness obtained was as follows : Direct light 100 Silvered mirror 9 Quicksilvered glass 76 Metallic mirror, one reflection Herschel gives also : Newtonian telescope 44 Gregorian or Cassegrainian 40 Steinheil : Object glass by Fraunhofer transmits 76 Object glass by Steinheil 87
