Ocular refraction and the shadow test . oncavescopic; C and D, bi-concave iicave, meniscus spherical lenses, they are termed concaii ^pluinal lenses. Itwill be seen that the parallel rays R in palssing through the concavelens L are refracted as divergent rays R and therefore can nevermeet, so that they are not brought to a focus. A concave lens there-fore has no real fccus but only a inrtiial focus which is located by ex- tending the divergent rays back to a point where they would meet,as P figure 54. Various types of concave spherical lenses are illus-trated by figure 55; A is a plano-concave
Ocular refraction and the shadow test . oncavescopic; C and D, bi-concave iicave, meniscus spherical lenses, they are termed concaii ^pluinal lenses. Itwill be seen that the parallel rays R in palssing through the concavelens L are refracted as divergent rays R and therefore can nevermeet, so that they are not brought to a focus. A concave lens there-fore has no real fccus but only a inrtiial focus which is located by ex- tending the divergent rays back to a point where they would meet,as P figure 54. Various types of concave spherical lenses are illus-trated by figure 55; A is a plano-concave, B a convexo concave, orperiscopic, C and D double or hi concave. It will not be necessaryto go all over the characteristics of the concave lens as they are simi-lar to those of the convex, but of the opposite effect. A few wordshowever will not be amiss. They are thin in the centre, thicker at theperiphery, possess the same faults of aberration and distortion, if theyare rotated upon their centre they do not distort the object viewed. Appea Figure 56. : of an object Ihrougl] a through them. They are centred or decentred in the same way. Theeffect created by looking through such lenses is to make the objectappear smaller; figure 56 shows a figure I of a certain size and form,II shows this same figure seen through a concave lens at a given dis-tance, III the same figure with the lens fuither removed,showingthatthe greater the distance of the object from the lens the smaller it willappear to be. Repeat experiment illustrated by figure 50, using a concave lensand no difference will be detected between the convex and concave;however, if experiment illustrated by figure 48 be made with a con-cave lens the results will be shown by figure 57. The lens beingmoved to the right, the portion of the line vS L seen through tne lens L E N S E S. 53 also appears to the right of the line S L or with the movement of thelens. A convex lens can be distins^nished from a concave, by the
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