Ocular refraction and the shadow test . Figure 130. Emergent rays Irom an emmetropic eye brought to a focus imetre, by a convex spherical lens of one dioptre. Suppose it is desired to converge the emergent parallel raysshown in figure 129, so they will focus at the position of the observer40 inches distant. As it is known that a convex spherical lens of one dioptre willbring parallel rays to a focus at forty inches, if such a lens be placed. Emergent rays from an eye, myopicdistance of one metre, by ; before this emmetropic eye, as illustrated by figure 130, the emergentrays will focus at the
Ocular refraction and the shadow test . Figure 130. Emergent rays Irom an emmetropic eye brought to a focus imetre, by a convex spherical lens of one dioptre. Suppose it is desired to converge the emergent parallel raysshown in figure 129, so they will focus at the position of the observer40 inches distant. As it is known that a convex spherical lens of one dioptre willbring parallel rays to a focus at forty inches, if such a lens be placed. Emergent rays from an eye, myopicdistance of one metre, by ; before this emmetropic eye, as illustrated by figure 130, the emergentrays will focus at the observers eye O. The dotted lines representthe emergent rays, and show that their direction is changed fromparallel to convergent by the one dioptre convex lens interposed. If the eye under observation, shown in figure 130 were myopic to R E T I N O S C O P V. 163 one dioptre, the emergent rays would focus at the]observers eye atforty inches distance without interposing any lens. Let figure 131 represent an eye that is myopic to four dioptres,the emergent rays woull focus at a distance of ten inches, its farpoint. If the observer with the retinoscope were forty inches away,in order to bring them to a focus at this position, it would be necessaryto impose a concave spherical lens of three dioptres. This lens isseen to render the em jrgent rays less convergent, as shown by thedotted lines, figure 131. The conditions are now similar to
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