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 visual rays. At the Yerkes Ob servatory of the University 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 affected 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 the instru ment known as the cveatorial could 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 instrument, which has both visual and photographic object glasses 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 02 feet, weighs when mounted in its cell about 1000 pounds. The crown-glass lens, which is inches thick at the centre and 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 inches thick at the centre and 2 inches thick at the edges. The lenses are mounted upon alumi num bearings in a cast-iron cell. The telescope itself is mounted on a east-iron column of four sections bolted together and resting on a cast iron foot, which in turn rests on a concrete foundation. The clock room is located in the upper part of the cast-iron column 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 197 feet in length. Instead of be ing mounted equatorially. as are the large tele scopes at the Lick and Yerkes Observatories, t-he 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 the direct rays from the heavens fall. The side rostat, which was invented by Foucault, is a movable mirror with a plane surface mounted on such a mechanism that the light falling on the mirror from a given portion of the heavens will always be reflected to the object glass of the telescope. In other words, instead of the telescope being moved about its polar axis, as would happen with an equatorial, the mirror is given such a motion that it neutralizes the move ment of the earth. This idea is said to have been suggested by Robert Hooke at a time when the long-focus lenses of Huygens were being used and the difficulties attending their mounting were under consideration. On the occasion of the transit of Venus in 18S2 Lord Lindsay em ployed a heliostat in connection with his 40-foot telescope, while for taking photographs the Amer ican observers used the same method most suc cessfully. When it was proposed to erect at the Paris Exposition a telescope larger than any previously attempted, it was realized that an equatorial of greater size than those of the great observatories in the United States was prac tically impossible, and accordingly the fixed form was adopted. Both visual and photographic ob jectives are provided, and with the latter the largest photographs of the moon yet produced have been taken. Each lens weighs half a ton and they were constructed by 11. Gautier, who also made those for the University of Chicago telescope. Although in this form of telescope there is no elaborate mounting with mechanical devices, yet as much care must be expended on the side rostat as on the object glasses. The mirror form ing the essential part of this instrument is inches in diameter, is 11 inches in thickness, and weighs 3600 kilograms (nearly 8000 pounds).
It is believed by astronomers that for direct visual observations the refractor is the better instrument, while for the study of stellar, plane tary, and nebular spectra and the photography of these bodies the reflector is the more useful. Also the latter instrument can be used to advan tage in photometric and radiometric investiga tions of the moon and the planets. The reflector is free from chromatic aberration, and there is not the absorption of light which occurs when lenses of large apertures are employed. Larger angular and linear apertures can be used and the mount ing is far more simple than for a refractor of corresponding size. The development of the fixed refracting telescope is at present being awaited by astronomers, as it will obviate many difficul ties of construction, mounting, and expense.
Theory of Optics (London, 1902) ; Chauveuet, Spherical and Prac tical Astronomy, vol. ii. (Philadelphia, 1863) ; for history and theory, consult Lockyer, Stargaz ing, Past and Present (London, 1878). For more recent developments consult various arti cles in the Astrophysical Journal, especially by the following: Wadsworth, 1897 and 1902; Reese, 1902; Ritchey, 1901.