In reading, the myope is obliged to hold her book very close to the eyes in order to see. In doing so, she strains her muscles of convergence, producing ocular congestion and compression of the eyeball.
The predisposing causes of myopia are heredity; it is said that half of myopics are descended from near-sighted parents; uncorrected astigmatism, and the effort to read very fine print or figures, entails severe strain on the eyes, which may result in myopia.
Myopia is corrected by a concave lens, which diverges the rays of light, prolonging the focal distance, so that the rays of light are focused exactly on the retina.
Not only are myopic eyes not injured by wearing suitable glasses, but, on the contrary, are often preserved from injurious pressure on the globe, due to the indulgence of the habit to nearly close the lids in order to see better, as is commonly done when glasses are not worn.
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.
Fig. 12.—Astigmatic chart.