Ocular refraction and the shadow test . ated justinside of the principal focus; this gives to the emergent rays only aslight divergence. Behind the flame is placed a concave mirror thatreflects the light that strikes the back of the lamp, forward throughthe lens. The nearer the light is placed to the lens, the wider diver-gence is given to the emerging rays. Comparison of figure 120 with figure iis, shows that the emer-gent rays from the bicycle lamp, and those from a hypermetropic eyeare both divergent; therefore, the conditions of hypermetropia aresimilar to those of the lamp, viz:—the light
Ocular refraction and the shadow test . ated justinside of the principal focus; this gives to the emergent rays only aslight divergence. Behind the flame is placed a concave mirror thatreflects the light that strikes the back of the lamp, forward throughthe lens. The nearer the light is placed to the lens, the wider diver-gence is given to the emerging rays. Comparison of figure 120 with figure iis, shows that the emer-gent rays from the bicycle lamp, and those from a hypermetropic eyeare both divergent; therefore, the conditions of hypermetropia aresimilar to those of the lamp, viz:—the light source is inside the prin-cipal focus of the refracting system. 156 OCULAR E F R A C T I O N. Another comparison may be drawn between this lamp and theeye. Looking into the aperture of the lens, it appears black until thelamp is lighted, when it appears luminous. Looking into the aper-ture of an eye it looks black until light is projected upon the retina,which reflects a portion of it outward through the pupil, rendering Emergent parallel rays brought At this point the student jure 122. focus by interposing a convex spherical lens,advised to refer to figures 3S, 39, and4i,_ to refresh his memory upon the principles they illustrate; viz:—principal focus and conjugate foci. According to the laws of conjugate foci, the following experi-ments may be made. Let figure 121 represent a convex spherical lens of eight dioptres(+ D. S.), with a lighted candle placed at its principal focus,five inches. The light rays strike the lens divergent and emergefrom it parallel. If it is desired to bring these parallel emergent raysto a focus again at any certain distance from the lens, say ten inches,without changing the relative positions of the candle and lens.
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