. On the anomalies of accommodation and refraction of the eye, witha preliminary essay on physiological dioptrics. f the eye. We closed the preceding section with a practical result, namely :that accommodation can be maintained only for a distance, when, inreference to the negative, the positive part of the relative range ofaccommodation is tolerably great. In connexion with this point it is of special importance to show,that the relative range of accommodation in ametropic eyes is some-thing quite different from that in emmetropic. The difference is ofa twofold nature. In the first place, wit
. On the anomalies of accommodation and refraction of the eye, witha preliminary essay on physiological dioptrics. f the eye. We closed the preceding section with a practical result, namely :that accommodation can be maintained only for a distance, when, inreference to the negative, the positive part of the relative range ofaccommodation is tolerably great. In connexion with this point it is of special importance to show,that the relative range of accommodation in ametropic eyes is some-thing quite different from that in emmetropic. The difference is ofa twofold nature. In the first place, with a given convergence, therelation of the positive to the negative part of 1 : kx is not the same;in the second, the lines p1 p2 p and r r, have another form. We shall first treat of the relation of the positive to the negativepart of 1 : We may also thus express what we have said onthis point: the relative range of accommodation has, in reference tothe refraction of the eye, a totally different position with reference tothe line of the convergences k Ic. Fig. 60 illustrates this in detail. This Fig. 0° 11°21 22°50 34°32 46°38 59°20 72°50/ 120 RANGE OF ACCOMMODATION diagram contains the curves of the nearest and of the farthest pointsin a myopic eye M, and in a hypermetropic H. Both requirefurther explanation. First as to M. The beginning of the line r rx shows, that we have to do with a myopia of — ; the farthest bino- cular point r2 is found at 5. Up to this distance, that is up to aconvergence of about 28°, 1 : Ai is altogether positive. Now, how-ever, a negative part is rapidly developed, which even at 34°amounts to half the positive. So far 1: Ax was always increasing, 3and here attains as a maximum, --, while the total range of accommo- 7dation amounts to 1: A = — . Henceforth, however, 1 : A1 di- minishes a little; but the negative part becomes, meanwhile,greater and greater, and indeed up to about 50°, where, thedifficulties of convergence i
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