. On microscopical manipulation : being the subject matter of a course of lectures delivered before the Quekett Microscopical Club, January-April, 1869. 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 mu
. On microscopical manipulation : being the subject matter of a course of lectures delivered before the Quekett Microscopical Club, January-April, 1869. 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 practicaloptician, and are unsuitable for an elementary treatiseon the microscope. 9- Fig. Red. Chromatic aberration is caused by the unequal re-fraction of the constituents of a ray of light. If aray be bent bypassing through a glass prism, insteadof the ray appearing on the other side as white lightit will be decomposed, owing to the unequal capabilityof bending which the rays possess ; the red ray is bentleast, the yellow more, and the blue and violet mostof all (Fig 9). This also takes place when light iscaused to pass through a lens (Fig. 10). The blue raybeing the most refrangible comes to a focus nearestto the lens at b; the red ray being the least refrangiblewill not converge until it reaches r; the yellow, green,&c., rays come to a focus at intermediate points, sothat a series of images surrounded by fringes of their i5 MICROSCOPICAL MANIPULATION. respedtive colours would be formed at various pointsbetween b and r, such coloured margins renderingthe definition extremely defective. Ch
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