Elements of natural philosophy (Volume 2-3) . l 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 lens, the combination will be of the form usedby Galileo, who invented this instrument in 1609. Inthis construction, the eye lens is placed in front of theimage
Elements of natural philosophy (Volume 2-3) . l 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 lens, the combination will be of the form usedby Galileo, who invented this instrument in 1609. Inthis construction, the eye lens is placed in front of theimage at a distance equal to that of its principal focus, sothat the rays composing each pencil shall emerge from itparallel. Draw through the point p, where the image of Pwould be formed, the line p 0, to the optical centre 0 ofthe eye lens, and through the optical centre 0 of the eye,the line 0 ^parallel to^> 0, its intersection K, with theretina will give the image of the point P on the backpart of the eye. ELEMENTS OF OPTICS 247 Fig. Galileantelescope; The rule for finding the magnifying power of this Magnifyinginstrument is the same as in the former case: for we pov;er fo™d analytically; have, which in Equation (67), after making/, and d, negative,gives A 11- (73) Ratio °f visual angles; and for parallel rays, A F, U m . . (74) Same for parallelrays; The second member being positive, shows that objects objects appearseen through the Galilean telescope appear erect. § 78. If we divide both numerator and denominator ofEquation (72), by Fu. (F„), it becomes, A A m i~ F. n Magnifyingpower in termsof the powers ofthe lenses; and denoting by Z, the power of the field, and by l% thatof the eye lens, we have A^A L • • • • (75) Ratio of visualangles; 243 NATURAL PHILOSOPHY. Rule for magnifying power. that is, the magnifying power of the astronomical tele-scope is equal to the quotient arising from dividing thepower of the eye lens hy that of the field lens. Fig. 51
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