Reviews : On the anomalies of accommodation and refraction of the eye, with a preliminary essay on physiological dioptrics, by F.C. Donders ... translated from the author's manuscript by William Daniel Moore ... The New Sydenham Society, Vol. XXII., London, 1864.

Date:
[1865]
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Credit: Reviews : On the anomalies of accommodation and refraction of the eye, with a preliminary essay on physiological dioptrics, by F.C. Donders ... translated from the author's manuscript by William Daniel Moore ... The New Sydenham Society, Vol. XXII., London, 1864. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    the axes of the eye and lens coincide.* The power of a lens, more- over, is inversely proportioned to its principal focal distance ; and it is expressed by the reciprocal of the latter. Thus, in the present case where the focal distance of the imaginary lens is = H C, the power may be taken as which was the expression already Jd C HO obtained for the range of accommodation in the emmetropic eye, with its nearest point at C. In like manner, an auxiliary lens, which, when placed at A H, would deflect the ray D A in the direction C A, or the ray B A in the direction E A, would represent by its refracting power the range of accommodation in the case of the myopic and hypermetropic eyes, respectively; and thus the angles, A D H - A C H, and AGH, represent the power of the lenses required as substitutes for accommo- dation in these two cases. These angles are to one another as iTI) — ITc *s 5o~~Ah as we have alre&dy seen. If we represent the focal distance of the lens, required in any case by A, its refracting power is represented by the reciprocal d; this xx also represents the range of accommodation; and we have now the equation — = -p — p-. This general expression is much used by Bonders in the course of his work, ~ being the symbol employed by XX him for the range of accommodation, and d—JL being its equivalent. If we suppose the distances H C and HG to be, each of them, 8 inches, and the distance H D to be 4 inches, the ranges of accom- 111 1 modation represented by g-j-—and are all equal to ene another; for they may be expressed thus |, ^ — ■§ = ■§, and |; but the regions differ very much in extent. The emmetropic eye in our illus- tration can accommodate for any distance greater than 8 inches ; its region is therefore infinite. The hypermetropic eye has also a region in- finite in extent, though differing in position from that of the emmetropic. * Because the accommodative change in the one case, and the lens in the other, sufficed to deflect a ray coming from the nearest point into the direction which it would have pursued had it come from the farthest point; and because all rays whose direction-lines, before refraction, intersect or are parallel to the axis of the same lens, will be deflected in the same degree by refraction at the lens, provided the refraction takes place in each case at the same distance from the axis, and provided that the lens is infinitesimally thin.