. On microscopical manipulation : being the subject matter of a course of lectures delivered before the Quekett Microscopical Club, January-April, 1869. e image formed by the small lensor objedt-glass. These imperfe(5tions are chieflycaused by the spherical and chromatic aberrations ofthe objedt-glass, and also in some degree by those ofthe eye-piece. Theoretically, convex lenses bring parallel rays to apoint called the focus, at a distance from the lensdependent upon the radii of their curved , this is not the case; the rays passingthrough the marginal portion of the lens com
. On microscopical manipulation : being the subject matter of a course of lectures delivered before the Quekett Microscopical Club, January-April, 1869. e image formed by the small lensor objedt-glass. These imperfe(5tions are chieflycaused by the spherical and chromatic aberrations ofthe objedt-glass, and also in some degree by those ofthe eye-piece. Theoretically, convex lenses bring parallel rays to apoint called the focus, at a distance from the lensdependent upon the radii of their curved , this is not the case; the rays passingthrough the marginal portion of the lens come to afocus at a shorter distance from it than those which 14 MICROSCOPICAL MANIPULATION. pass through or near its centre (Fig. 8). Thosepassing through intermediate portions converge tosome part between these two points ; the result isthat indistindt images are formed at various pointsbetween f and/, Fig. 8, no well-defined figure beingseen anywhere. Could lenses be made having anhyperbolic or elliptic sedlion, the spherical aberrationwould be corrected, but the difficulties of makinglenses of such figures are so great as to be consideredFig. impossible in practice. Spherical aberration maybelessened by reducing the aperture of the lens, bycutting off the marginal portion by means of stops ordiaphragms ; but any great use of this means causesa loss of light very detrimental to the performanceof the instrument. Advantage may also be taken ofthe position in which the lens is placed and also ofits figure; the spherical aberration of some lenses,such as plano-convex and meniscus, is much affedtedby the way they are placed, being much greater inone position than other. Much may be done byplacing the glasses in their most favourable positionand using suitable curves : as a striking example, take CHROMATIC ABERRATION. 15 to pieces a photographic lens and observe the curiousforms adopted in its construction. The details ofthese corrections are rather a matter for the practicalopti
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