Light; a course of experimental optics, chiefly with the lantern . t beyond^^ principal focus,they must converge to some other point and form an image. ^ A lens formed by two equal convex surfaces is called a doubleconvex lens, and if the two convex surfaces are of different curves,a crossed lens; lenses with two concave surfaces are double-concavelenses; those with one side flat, plano-convex or plano-concave ; a lenswith one side convex and one concave, a meniscus. 62 LIGHT. [CHAP. Fig. 46 shows this. The parallel rays from A and bconverge at the principal focus r after traversing the lenso;
Light; a course of experimental optics, chiefly with the lantern . t beyond^^ principal focus,they must converge to some other point and form an image. ^ A lens formed by two equal convex surfaces is called a doubleconvex lens, and if the two convex surfaces are of different curves,a crossed lens; lenses with two concave surfaces are double-concavelenses; those with one side flat, plano-convex or plano-concave ; a lenswith one side convex and one concave, a meniscus. 62 LIGHT. [CHAP. Fig. 46 shows this. The parallel rays from A and bconverge at the principal focus r after traversing the lenso; but if rays diverge from the points a and E of an object(only pairs of rays are shown for the sake of clearness) theyconverge to the points a b and form an image. It is alsoclear how the respective distances of image and objectgovern their respective sizes, so that \i a b is , a bwill be its image. Also that the image thus formed mustbe inverted. Thus we have a second method of forming brilliantimages, the lens taking up a large cone of rays from each. Pig. 46.—Image formed by Lens. point of the object. As in the case of mirrors, however, thespherical surfaces which are most easily ground do nottruly converge the rays to a point, except for a smallcentral portion of the lens ; the figure necessary to do thisbeing parabolic. Such parabolic lenses have been ground,though with great difficulty. There are however other errorsalso to be corrected; and it is easier and more convenientto correct all these errors by methods presently described,or to stop off some of the most erroneous marginal rays. 39. Virtual Images and Foci.—A mere inspectionof Fig. 47 will show that if the object or luminous point in.] VIRTUAL IMAGES AND FOCI. 63 be nearer the convex lens than its principal focus f, therays cannot form an image, but simply become lessdivergent. If the emergent ray-lines are produced back tos, that will be the virtual focus. In this case we have
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