Elements of natural philosophy (Volume 2-3) . focal dis-tance of the second reflector, 1 11 Same expressed t = Tf> ? ~ analytically; whence / F2 c c = F2-c dividing by — o, c JT A Same in other terms; F„-c~ A 25G NATURAL PHILOSOPHY. which, being multiplied by Equation (80), member bymember, gives for the magnifying power of this tele-scope, Magnifyingpower ofGregoriantelescope; May be varied. Objects appearerect. AA F. Fn m Fa—c . (SI) whence, this ratio being positive, the object will appearerect; its apparent magnitude may be made as greatas we please by giving a motion to the small refle
Elements of natural philosophy (Volume 2-3) . focal dis-tance of the second reflector, 1 11 Same expressed t = Tf> ? ~ analytically; whence / F2 c c = F2-c dividing by — o, c JT A Same in other terms; F„-c~ A 25G NATURAL PHILOSOPHY. which, being multiplied by Equation (80), member bymember, gives for the magnifying power of this tele-scope, Magnifyingpower ofGregoriantelescope; May be varied. Objects appearerect. AA F. Fn m Fa—c . (SI) whence, this ratio being positive, the object will appearerect; its apparent magnitude may be made as greatas we please by giving a motion to the small reflectorwhich shall cause^ its principal focus to approach thefirst image, and drawing out, at the same time, the eyelens to keep the second image in its principal focus. Casscgrainiantelescope ; § 84. If the small reflector be made convex insteadof concave, we have the modification proposed by , and called the Cassegrainian telescope, whichis represented in the figure. Its magnifying power is Fig. 56. Graphicrqiivsentation;. given by Equation (81), by changing the signs of Faand <?, which will give, Magnifyingpower; AT_A F. F. TO c-F, ? (82) Objects appearinverted. and because the ratio of A to A is negative, objectsseen through this telescope with ordinary eye pieces,appear inverted. ELEMENTS OF OPTICS. 257 § 85. Sir Isaac Newton substituted for the small cur- Newtonianved reflector a plane one inclined under an angle of 45° telescope;to the axis of the instrument, and so placed as to inter-cept the rays before the image is formed. The vergencynot being affected by reflexion at plane surfaces, § 55,the image is formed on one side, and viewed throughthe lens supported by a small tube inserted in the side of Fig. 57.
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