. Good and bad eyesight : and the exercise and preservation of vision. 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 me
. Good and bad eyesight : and the exercise and preservation of vision. 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 less exactness, the convergence of paral-lel rays to a focus; and by making the dense medium. convex on both sides, both conspire to produce thedesired effect. The distance of the focus behind the medium dependson the refracting power of the substance employed, andon the degree of convexity of its surfaces. The greaterthe convexity of the two surfaces, and the greater therefractive power, the nearer will be the focus. We have next to apply the foregoing principles tothe formation of images by lenses; and for this pur-pose it is necessary first to consider the results of theordinary transmission of light in straight lines fromits source. If we take a card, as A, Fig. IO, perforateit by a central hole about one-tenth of an inch in 40 EYESIGHT. diameter, and then place it between a lighted candleand a white screen, in an otherwise darkened room, weshall see an inverted image of the candle-flame uponthe screen. The light, radiating from the flame inevery direction, strikes partly upon the perforated partof the card, and is thereby either reflected or absorbed •but that which pas
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Keywords: ., bookcentury1800, booksubj, booksubjecteye, booksubjecteyediseases
