. On microscopical manipulation : being the subject matter of a course of lectures delivered before the Quekett Microscopical Club, January-April, 1869. y using two, one of larger dimen-sions than the other, the light being brought to acertain degree of convergence by the first and largerlens, and afterwards concentrated by passing throughthe smaller one, which should be so placed as tointercept the cone of light from the large lens at theplace where it is of its own diameter. Most of the other illuminators of this class adt byrefledtion, mirrors of various forms being optical pro
. On microscopical manipulation : being the subject matter of a course of lectures delivered before the Quekett Microscopical Club, January-April, 1869. y using two, one of larger dimen-sions than the other, the light being brought to acertain degree of convergence by the first and largerlens, and afterwards concentrated by passing throughthe smaller one, which should be so placed as tointercept the cone of light from the large lens at theplace where it is of its own diameter. Most of the other illuminators of this class adt byrefledtion, mirrors of various forms being optical properties of concave mirrors resemble,in many points, those of convex lenses; they causeparallel rays to converge (Fig. 19), render divergent Fig. 19. rays parallel, or convergent, and also form images(Fig. 20). Microscopes and telescopes can be con-stru(5ted with mirrors or specula, instead of objedl-glasses ; this principle is still in use as regards tele-scopes of large dimensions, as the difficulties of 95 MICROSCOPICAL MANIPULATION. making objedl-glasses of large diameter are very great; the largest yet accomplished is only 25 inches, Fig. while the largest speculum (that of the late LordRosse) is 6 feet in diameter. Professor Amici constructed a very efficient com-pound microscope, having an elliptical speculum inplace of an obje6t-glass ; the plan, however, wasabandoned on the introdu6tion of the achromaticprinciple, although its performances were verysuperior to those of the non-achromatic instrument.*Concave mirrors, when formed of segments of spheres,are liable to spherical aberration, just as lenses are(Fig. 21) ; therefore, when it is required to renderdivergent rays parallel, or bring parallel rays to afocus, it is necessary that the figure of the mirrorshould be that of a parabola. Specula of thisform are used in astronomical telescopes, therefledlors of lighthouses on the catoptric principle,and in some instruments presently to be parabolic curve
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