In old age the power of accommodation of the eye is diminished. The ciliary muscle becomes weakened, so that it loses its ability to increase the thickness of the crystalline lens by compression. The result of this change is that the individual is unable to see near objects as well as formerly. In reading, he is obliged to hold his book or paper farther away from the eye than usual. Objects at a distance are seen as before, the difficulty being only observed with reference to near objects. By placing a convex lens, Fig. 450, before the eye, the deficient power of the crystalline lens is compensated for, and the patient can see near objects without difficulty, but is obliged to remove the glasses when viewing distant objects. By some means, the process known as accommodation, by which the eye is adapted to view objects at different distances, which the eye becomes incapable of performing in old age, may be imitated by the use of artificial lenses. Old people who are able to see without glasses, generally have an unnaturally long eyeball, in consequence of which their far-sight is deficient, although they may have excellent vision for near objects. Old people are sometimes agreeably surprised by finding themselves able to read without glasses after they have been obliged to use them for many years. This is what is know as second sight, which results in a change of the cornea by which the eye is made short-sighted.

Fig. 450. Convex Lens.

Fig. 450. Convex Lens.

As age advances, the eye should be occasionally tested, especially if the individual finds that the eyes are tired more readily than usual by reading or use in fine work. Upon testing with the test types, if he finds that diamond type is most easily read at more than twenty inches from the eye, while number I can readily be made out only at a distance of fifteen or sixteen inches, he may be sure that his eyes are becoming presbyopic, and that proper glasses should be adjusted. It is a mistake to suppose that old-sighted persons can see better at a distance than persons with natural vision; hence, the term far-sighted, as applied to persons suffering with presbyopia, is incorrect. Old-sighted people see better at a distance than near by, but no better than those whose eyes are perfectly normal. Short-sighted persons do not generally require the aid of glasses nearly as soon as others, often in fact, getting along without them altogether.