. Good and bad eyesight : and the exercise and preservation of vision. actual deviation from parallelismis insensible. Let A, b, c, d, e, Fig. 8, represent theserays. Only one of them, c, by continuing its straightcourse, can arrive at the point f. The surface of thedense medium should be presented at right angles tothis ray, at i, so that the ray may pass through themedium without deviation. Those rays, b and d,which are situated near to the direct or central ray c,will require but a small degree of refraction to enablethem to reach the focus f. and this small refraction will 38 EYESIGHT. be
. Good and bad eyesight : and the exercise and preservation of vision. actual deviation from parallelismis insensible. Let A, b, c, d, e, Fig. 8, represent theserays. Only one of them, c, by continuing its straightcourse, can arrive at the point f. The surface of thedense medium should be presented at right angles tothis ray, at i, so that the ray may pass through themedium without deviation. Those rays, b and d,which are situated near to the direct or central ray c,will require but a small degree of refraction to enablethem to reach the focus f. and this small refraction will 38 EYESIGHT. be produced by a small degree of obliquity in the densemedium at the points h and k. In proportion as therays A and E are more distant from the central ray, agreater amount of refraction, and consequently a greaterobliquity of surfaces at ^and /, will be required to bringthem to the same focus. On the presumption that therays passed through a medium of uniform density, theywould converge to a focus, then, at f. Let Fig. 9 represent the same rays a, b, c, d, e, Fig. entering the same medium as before, but instead of themedium being continued, let it be supposed to terminateat the curved surface m, n, oy p} q, so that it now formsa double convex lens. The central ray c proceeds atright angles through both surfaces, and reaches Fr orf, without deviation. The rays b, d, a, e are re-fracted towards the perpendiculars on passing into thedense medium at the points hy k, g, /, but on quitting itthey are refracted from the perpendiculars to the surfaceof the rare medium, at the points ?t, p, m, q. This new THE FORMATION OF IMAGES. 39 refraction increases the convergence of the rays, andbrings them to a focus f, nearer to the dense mediumthan the former focus f. The result of the continual change of direction in therefracting medium is a regular curvilineal surface,approaching to the spherical. By giving such surfacesto refracting substances they become fitted to produce,with more or le
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