The magnifying power of telescopes is usu ally expressed in diameters and is indicated by the ratio of the focal length of the objective to that of the combination of lenses forming the eye-piece. For example, the Lick telescope has a focal length of 56 feet or 672 inches. If, therefore, an eye-piece of one inch focus is used power of 100 diameters is four times as bril liantly illuminated as would be the case if an eye-piece giving 200 diameters were used. It follows, therefore, that the observer will use the power best adapted to his purpose, both as to magnification and light. • The telescope giving the minimum power is the opera glass, usually magnifying two and one-half or three diameters, which is sufficient for indoor use, while for outdoor use the Gali lean binocular has a power of four or five diameters, and the prism binocular of six, eight, 10 or even 12 diameters. The eight power is, however, considered as high as can be held in the hands with sufficient steadiness to give the best results. The power used in terrestrial telescopes steadily mounted on a tripod usually ranges from 15 to 100 diameters, depending on the condition of the atmosphere.
The most important element in a refracting telescope is the objective, and, in a reflecting telescope, the mirror or speculum. The objec tive of the early refracting telescope was a double-convex lens, which could not give a dis tinct image because it separated each ray of light into its various prismatic colors, and each color, having a refracting power different from tbe others, found a focus of its own. The red ray, being most refracted, reached its focus first; next came the yellow ray, then the green and last and nearest of all to the eye-piece, the blue ray made its image. When a star is viewed with such a telescope the image seen consists of a yellowish point at the centre, sur rounded by a mixture of green and blue light, with red outside. This difficulty in refracting telescopes, called chromatic aberration, checked the progress of astronomy for 150 years, until, in 1750, the English optician, Dolland, invented the achromatic objective as shown in Fig. 1. In this diagram E is a double-convex lens of crown glass and D a flint-glass lens of nearly plano-concave forin. The difference between the crown and flint material in light refraction and dispersion, together with the compensating form of the two lenses, results in clear and distinct gclefinition,)) the image of the star being sharply outlined and colorless. Optical glass for such lenses is of special manufacture. The world's supply comes from three makers, one each in England, France and Germany. The
newer objectives are made. of several thin len ses cemented together, usually four in number and alternately crown and flint glass. Some makers are using five lenses successfully, though which much difficulty. The mirror or speculum of the reflecting telescope is made from a casting of ordinary glass of sufficient thickness to be handled without flexure, pref erably one-sixth of the diameter of the disc, although one-eighth is a commonly accepted pro portion in the case of very large discs. The reflecting surface is ground and polished, with great precision, to a parabolic form of the focus required, and then, by a chemical process, coated with silver, which may be easily renewed when tarnished or otherwise injured.
The making of the optical parts of tele scopes is a rare art, which, however, has been cultivated with peculiar success in America. Alvan Clark and Sons of Cambridge, Mass., attained world-wide fame in this connection during the lifetime of the gifted men composing the firm. At the present time the largest repu tation as a maker of large telescopic objectives belongs to John A. Brashear of Pittsburgh, Pa.
The telescope tube is usually carried by an equatorial mounting. This form of instrument has its principal or polar axis set parallel to the axis of the earth, its inclination, therefore, corresponding to the latitude of the observa tory. At right angles to the polar axis is the declination axis, which, in turn, carries the telescope tube at right angles to itself. Each axis is supplied with a graduated circle, indi cating, respectively, the position of the star in hours, minutes and seconds of right ascension, and in degrees, minutes and seconds of declina tion. It will be evident that when the tube is pointed to a star in any part of the visible heav ens, a revolution of the polar axis from east to west, in sidereal time, will malce the telescope follow the apparent motion of the star. A driv ing clock, which is usually located inside the column of the instrument, controls the revolu tions of the polar axis so that the star observed remains steadily in the field of vision. The equatorial principle has been applied to photo graphic telescopes in such manner as to allow the continuous exposure of the photographic plate during the entire night, if desired. One of the most ingenious forms of mounting is the Equatorial Conde. In this instrument the polar axis is enlarged so as to serve as the main tube of the telescope, the eye-piece being at the upper end where the observer can sit comfortably in his warm room and observe any star in the visible heavens as easily as he uses his mi croscope.