TELESCOPE. 108 TELESCOPE. which is composed of a mixture of copper and tin, until Liebig discovered the method of de- positing a film of silver on a glass surface. The use of silvered glass for mirrors was sug- gested bj' Steinheil and later by Foucault, and has met with general adoption, as it not only fa- cilitates the construction of the mirror, but makes possible its resilvering at any time with- out the destruction of its configuration. The reflecting telescope is available for photographic or spectroscopic work, as well as for visual ob- servation, but the chief defects are the difficulty of grinding the metal or glass to true parabolic shape and the deformation of the mirror in it3 mounting owing to its great mass. In recent years the most important use made of a reflect- ing telescope has been to photograph the Nebulse with the Crossley reflector of the Lick Obser- vatory (q.v. ), which is the largest telescope of this description in the United States. It was made by Dr. A. A. Common and was presented to Lick Observatory by Edward Crossley. Pro- fessor James T. Keeler, the director, remounted this instrument and in 1899 made a remarkable series of photographs. The mounting of telescopes has also kept pace with the improvement of lenses and mirrors. Newton used a ball and socket joint to mount his small reflector, and then various arrange- ments of framework were employed for the aerial telescopes and later for the large reflectors. The first equatorial (q.v.) mounting is ascribed to Lassel. In England telescopes were mounted by having the polar axis supported at each end, but the German system, where the mounting is in the centre and the weight of the telescope is balanced by counterpoises, is now generally used for large refractors. The modern telescope is not merely an instrument for visual observation, but in con- nection with photography and the use of the spec- troscope its field has been widely increased. Fraunhofer, who first used the spectroscope in- stead of the visual eyepiece, was able to record the spectra of Venus and Sirius, and since that time the combination of telescope and spectro- scope has made possible some of the most im- portant advances in astronomy. One of the earli- est applications of photography was to the heav- ens, and the object glass of the telescope was used with the eyepiece removed, the image being formed directly on the plate. In the early stages of this work both reflecting and refracting telescopes were used, but with the latter it was found that special lenses were necessary in which the correction for chromatic aberration was ar- ranged with regard for the actinic rather than the visual rays. The green, yellow, and red rays, which affect the human eye the most, do not pro- duce the chemical effects on the photographic plate, which must be placed at the focus of the violet rays, and consequently a photographic ob- jective is constructed so as to bring the blue and violet rays to a single focus. One of the first telescopes constructed for this purpose was by Lewis M. Rutherfurd (q.v.), with which many fine pictures of the moon were made. At the Lick Observatory photographic work with the large telescope has been accomplished most suc- cessfully by using an extra lens at the objective, which brings the photographic rays to a focus instead of the visxial rays. At the Yerkes Ob- servatory of the ITniversity of Chicago this dif- ficulty has been overcome by the use of color screens, and photographs made in 1900 indicate the general usefulness of the method. The color screen is mounted in the plate-holder and is of a greenish yellow tint, which cuts off the blue rays. Orthochromatic plates are used and- are afl'ected by the rays in the middle part of the spectrum. So important is the photographic use of the tele- .scope that in 1900 there were 18 photographic telescopes in various parts of the world engaged in making photographs for an International Star Chart to cover the entire heavens. Of the modern telescopes of interest the instru- ment known as the equatorial coiide of the Paris Observatory is worthy of mention. In this in- strument the observer is comfortably situated in an ordinary building, while mirrors are so ar- ranged as to reflect the light into the telescope and then to the eyepiece. The tube of the telescope is in the form of an elbow, whence its name, with one arm forming the polar axis. At the point of intersection of the two arms there is placed a plane mirror, while in front of the object glass is a second plane mirror which is movable and enables light from any portion of the sky to be reflected into the tube. Very suc- cessful photographs of the moon have been made with this instnmient. which h,as both visual and photographic olijcct glasses 23V. inches in diam- eter. The great telescope of the Yerkes Ob- servatory of the University of Chicago at Will- iams Bay, Lake Geneva, Wisconsin, is the largest telescope with an equatorial mounting, and until the construction of the Paris Exposition telescope was the largest refractor in existence. This in- strument was mounted in a special observatory (for illustration, see Observatory) and both lenses and mounting represented the best work of modern opticians and mechanicians. The objective, which is 40 inches in diameter and has a focal length of nearly 62 feet, weighs when mounted in its cell about 1000 pounds. The crown-glass lens, which is 2 Vi> inches thick at the centre and Si inches at the circum- ference, weighs 200 pounds, and is separated from the flint-glass concave lens by 8% inches. The latter weighs over 300 pounds and is about 1% inches thick at the centre and 2 inches thick at the edges. The lenses are mounted upon alumi- nimi bearings in a east-iron cell. The telescope itself is mounted on a cast-iron column of four sections bolted together and resting on a cast- iron foot, which in tvirn rests on a concrete foundation. The clock room is located in the upper part of the cast-iron cohunn and contains the mechanism for driving the telescope. The instrument is in perfect balance, a mechanical feat of no little difficulty when it is remem- bered that there is a weight of 1000 pounds at the object-glass end. The observatory is provided with a rising floor for the convenience of the observer, who is thus able to use the telescope in comfort, irrespective of its elevation. The most re- cent as well as the largest telescope is that built for the Paris Exposition of 1900 by M. Gautier, which differs materially from the large telescopes previously constructed. The object glasses of this great instrument are 49 inches in diameter and the tube 107 feet in length. Instead of be- ing moimted equatorially, as are the large tele- scopes at the Lick and Yerkes Observatories, the Exposition telescope is fixed in a horizontal po- sition, its axis being due north and south. Light is reflected into the tube by a siderostat on which
