Elements of natural philosophy (Volume 2-3) . Refractingtelescope; The image p q, being in the principal focusof the lens m n, draw from the point p, the line Construction forp 0, to the optical centre of this lens; the rays from p n^uracecUowill, § 73, be deviated parallel to this line, and the line tbe retina-0 K, through the optical centre Of of the eye, paral-lel to p 0, will determine by its intersection K,with the retina, the place upon that membrane of theimage of the point P. Calling the principal focal distance of this lens, (F{) ;d, in Equation (67), will equal f + (Fn\ and that equa
Elements of natural philosophy (Volume 2-3) . Refractingtelescope; The image p q, being in the principal focusof the lens m n, draw from the point p, the line Construction forp 0, to the optical centre of this lens; the rays from p n^uracecUowill, § 73, be deviated parallel to this line, and the line tbe retina-0 K, through the optical centre Of of the eye, paral-lel to p 0, will determine by its intersection K,with the retina, the place upon that membrane of theimage of the point P. Calling the principal focal distance of this lens, (F{) ;d, in Equation (67), will equal f + (Fn\ and that equa-tion will become, by first making f and d negative andthen replacing d by this value, -4A f tf (K) General equation( i 1) made applicableto this telescope; and if the object P Q, be so distant that the rays com-jiosing each of the small pencils whose common base isMN, may be regarded as parallel, fn becomes Flt,and we have, A ZLW Ratio of visual. (72) angles for parallelrays; 246 NATURAL PHILOSOPHY. Fig. 49. Kefraetingtelescope;. Compoundmicroscope ; Field and eyelenses; Rule for magnifyingpower. Objects appearinverted. Galileantelescope; Construction ofimage on theretina ; Equation (71) involves the principles of the com/poundrefracting microscope, and refracting telescope / andEquation (72), which is a particular case of (71), relatesto the astronomical refracting telescope. The lens M iV,next the object, is called the object or field lens, and mn,the eye lens. The magnifying power in the first case, isequal to the distance of the image from the field lensdivided by the principal focal length of the eye lens;and in the second, to the principal focal length of thefield lens, divided by that of the eye lens. The ratio of A to A, being negative, shows that ob-jects appear inverted through these instruments, the vis-ual angles of corresponding parts of the object and im-age being on opposite sides of the axis. § 77. If instead of a convex, a concave lens be usedfor the eye len
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