Another abnormality is "long sight." In the hypermetropic eye, parallel rays of light are brought to a focus at a point beyond the retina, so that divergent or parallel rays cause diffusion circles and a blurred image. This may be corrected by means of convex glasses, which make the rays convergent before they strike the corneal surface, and thus enable them to be sooner brought to a focus by the dioptric media of the eye. Presbyopia is the name given to a change in the perfectness of accommodation frequently accompanying old age. The lens probably becomes less elastic and the ciliary muscle weaker, so that the change in form required to see near objects is difficult or impossible to attain. Biconvex lenses help to overcome the difficulty.
Fig. 227. Showing the course of the rays of light from two luminous points to the retina. The rays from the point a on passing through the cornea, lens, etc., are collected on the retina at b. Those from a' meet b', and thus the lower point becomes the upper.