. On the anomalies of accommodation and refraction of the eye, witha preliminary essay on physiological dioptrics. ecided dispersion. Tor ophthalmologicalpurposes, on the contrary, only slight declination is required, and wetherefore use prisms with a smaller angle of refraction, from 3° to24°; moreover, it is necessary to select a kind of glass, which, withreference to its refracting power, presents but slight dispersion, forexample, crown glass. Such prisms are given in the boxes forOphthalmological use, usually in sixteen numbers. The numbersfrom III. to XXIV. indicate the refracting angles
. On the anomalies of accommodation and refraction of the eye, witha preliminary essay on physiological dioptrics. ecided dispersion. Tor ophthalmologicalpurposes, on the contrary, only slight declination is required, and wetherefore use prisms with a smaller angle of refraction, from 3° to24°; moreover, it is necessary to select a kind of glass, which, withreference to its refracting power, presents but slight dispersion, forexample, crown glass. Such prisms are given in the boxes forOphthalmological use, usually in sixteen numbers. The numbersfrom III. to XXIV. indicate the refracting angles, 3° to 24°. Inthe position of least declination, the angles of declination, for thelow numbers, are nearly the half of these refracting angles : for thehigher numbers, they are somewhat more. If we wish to knowthese accurately, it is necessary to determine the deviations for eachglass separately. The declination of the light by prismatic glasses is the cause whyobjects, seen through such glasses, exhibit themselves in anotherdirection. Let i (Eig. 65) be a point of light, i a a ray falling at a Fig:. on one of the refracting surfaces of the prism, this approaches the per-pendicular v a, instead of continuing in the same direction a a, and runsas a b in the prism; arrived at b it again alters its direction, now in
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Keywords: ., bookcentury1800, bookdecade1860, bookidonanomalieso, bookyear1864
