Eyesight, and how to care for it / edited by George Black.

Date:: [1888]

Catalogue details

Licence: Public Domain Mark

Credit: Eyesight, and how to care for it / edited by George Black. Source: Wellcome Collection.

78/168 (page 74)

which is too near the cornea. If the shape of an emmetropic eye be compared to that of an orange, a hypermetropic eye will have more the form of a tur- nip. In Fig. XVI., the continuous outline shows the form of the emmetropic eye, and the dotted curve indi- cates the position of the retina in hypermetropia. It will be seen that rays of light which enter the eye par- allel are intercepted by the retina, in this false posi- tion, before they are brought to a focus, and that only convergent rays can be focused accurately upon it. As all rays of light that have not passed through a convex lens are either divergent or parallel, and con- vergent rays are not met with in Nature, it would seem, at first sight, that an eye with too short an axis could have no distinct vision at any distance. This would be the case if it were not for the power of accommo- dation, which enables the eye, by increasing the con- vexity of its lens, to bring the focus forward on to the retina. (See Figs. XIII and XIV.) This, however, as has been explained, is accomplished by muscu- lar action, and such an eye is subject to constant istrain; even distant vision, which in the perfect eye is a passive sensation consistent with entire rest of the organ, requires an effort. In reading, the effort required, even by the natural eye, must be added to that already in use for distance, in the case of the hypermetropic eye, and the strain is propor- tionately increased.