[Illustration: FIG. 97.—Hartness Turret Observatory from the N. E.] Circles arc a necessity in all but the small telescopes used on portable tripods, otherwise much time will be wasted in finding. In any event do not skimp on the finder, which should be of ample aperture and wide field, say ¼ the aperture of the main objective, and 3° to 5° in field. Superior definition is needless, light, and sky room enough to locate objects quickly being the fundamental requisites. As a final word see that all the adjustments are within easy reach from the eyepiece, since an object once lost from a high power ocular often proves troublesome to locate again. REFERENCES CHAMBERS’ Astronomy, Vol. II. F. L. O. WADSWORTH: _Ap. J._, =5=, 132. Ranyard’s mounts for reflectors. G. W. RITCHEY: _Ap. J._, =5=, 143. Supporting large specula. G. E. HALE: Cont. Solar Obs. # 2. The “Snow” horizontal telescope. G. E. HALE: Cont. Solar Obs. # 23. The 60 ft. tower telescope. J. W. DRAPER: Smithsonian Contrib. =34=. Mounting of his large reflector. G. W. RITCHEY: Smithsonian Contrib. =35=. Mounting of the Mt. Wilson 60 inch reflector. SIR H. GRUBB: Tr. Roy. Dublin Soc. Ser. 2. =3=. Polar Telescopes. SIR R. S. BALL: _M. N._ =59=, 152. Photographic polar telescope. A. A. COMMON: Mem. R. A. S., =46=, 173. Mounting of his 3 ft. reflector. R. W. PORTER: _Pop. Ast._, =24=, 308. Polar reflecting telescope. JAMES HARTNESS: _Trans. A. S. M. E._, 1911, Turret Telescope. SIR DAVID GILL: Enc. Brit., 11th Ed. Telescope. Admirable summary of mounts. CHAPTER VI EYE PIECES The eyepiece of a telescope is merely an instrument for magnifying the image produced by the objective or mirror. If one looks through a telescope without its eyepiece, drawing the eye back from the focus to its ordinary distance of distinct vision, the image is clearly seen as if suspended in air, or it can be received on a bit of ground glass. It appears larger or smaller than the object seen by the naked eye, in proportion as the focal length of the objective is larger or smaller than the distance to which the eye has to drop back to see the image clearly. This real image, the quality of which depends on the exactness of correction of the objective or mirror, is then to be magnified so much as may be desirable, by the eyepiece of the instrument. In broad terms, then, the eyepiece is a simple microscope applied to the image of an object instead of the object itself. And looking at the matter in the simplest way the magnifying power of any simple lens depends on the focal length of that lens compared with the ordinary seeing distance of the eye. If this be taken at 10 inches as it often conventionally is, then a lens of 1 inch focus brings clear vision down to an inch from the object, increases the apparent angle covered by the object 10 times and hence gives a magnifying power of 10.