From _myopia_ we pass to its opposite, _presbyopia_, or far-sightedness, (πρέσβυς, old, ὤψ, the eye,) this being a state of vision to which old age is so commonly subject. In presbyopia the refractive powers of the eye are feeble; that is, the rays of light are concentrated into a focus beyond the retina, and, therefore, too far back for the reception by the latter of a definite impression, at the ordinary distance, of normal vision. This condition may result from flatness of the cornea, from an insufficient convexity of the lens, or from a shorter degree than natural of the antero-posterior axis of the eye itself. The pupil is more or less contracted. The far-sighted person either removes the object under examination further than is usual from the eye, or he avails himself of convex glasses, the effect of which is, to increase the refraction, or convergence of the rays of light, before they enter the pupil. Although this condition of the eye is common in elderly or aged persons, we are not to suppose that it is confined to such alone. In aged persons it is one of those signs which bid man prepare for his last mortal change--a change, the premonitory warnings of which are beautifully described in the twelfth chapter of Ecclesiastes. But this affection sometimes occurs in the young; for a time it will remain stationary; but after a certain period, determined by various concomitant influences, it will greatly increase, till at length, indeed, it may be said, "those that look out of the windows be darkened." Another defect of vision may be next noticed; namely, _diplopia_, or double vision. This arises from various causes, some of them obscure, and, perhaps, connected with a morbid affection of the base of the brain, such as a tumor pressing on the _motor oculi_ nerve, an inflammatory condition of the brain and its membranes, or a sanguineous or serous effusion involving the origin of the third pair of nerves. It is often the accompaniment of squinting or strabismus, in which, although there is a want of correspondence in the movements or position of the two eyes, the vision of each singly is perfect. Sometimes, however, double vision occurs in one eye only, while, at the same time, there is no disturbance, as far as the harmony of the movements of the two eyes is concerned; occasionally, both eyes are affected by diplopia. In these cases, there must be some irregularity of refraction, either in the cornea or the lens.