_Hyperopia or Far-sightedness._—In this condition the eyeball is too short, and the rays of light from a distance are focused behind the retina. Instead of being distinct, the image is blurred. Hyperopia is corrected by a convex lens, which converges the rays of light, bringing them sooner to a focus. In the hyperopic eye the near-point of accommodation is at some distance, and a far limit of accommodation practically does not exist. _Presbyopia._—This is a loss of the power of accommodation, by which reading, writing, sewing, and other near work is accomplished. This power of accommodation is greatest in early life, and gradually diminishes until about the age of forty years, when reading at the ordinary distance becomes uncomfortable. At about seventy-five years of age the power of accommodation is practically lost. Every person over forty-five years, with normal or far-sighted eyes, should wear glasses to perform near work. _Astigmatism._—Astigmatism does not depend on the length of the eyeball, but on the curvature of the cornea, and rarely on that of the lens. Uncorrected astigmatism necessitates the expenditure of more muscular effort in the attempt to see distinctly than is necessary when refraction is normal. This is accompanied by early fatigue and more or less congestion of the vascular tunics of the eye. Astigmatism is corrected by a cylindric lens, which has a plane surface in one axis and a concave surface in the axis at right angles to it. [Illustration: Fig. 12.—Astigmatic chart.] In simple astigmatism, on looking at the accompanying astigmatic chart with each eye separately, certain lines in the defective meridian seem very much blurred, while those at exact right angles appear clear and black. This furnishes a test for astigmatism, since to the normal eye the lines appear of equal distinctness and clearness. Astigmatism is a very common ocular defect. =The Mechanism of Eye-strain.=—Comparatively few eyes are perfect. Far-sighted or astigmatic eyes can secure perfect vision by means of accommodation. By constant strain on the ciliary muscle, the crystalline lens is so increased in curvature as to exactly counterbalance the optical defect of the eyes. Healthy eyes should do their work without the consciousness of the owner, and this is a safe test as to the kind and amount of work demanded of them. Perfect rest for the eyes is impossible in the waking state—distant vision represents rest for the eyes and near vision exertion. Near work is the chief cause of near-sightedness. Distant vision should alternate with near work, and in near work the object should not be brought nearer to the eyes than 12 inches.