SIGHT, DEFECTS OF. Under this head we shall consider such affections of the eye sight as are due to some known or unknown peculiarity of the optical apparatus (includ ing the optic nerve) not dependent ou disease—viz., short-sight, long-sight, double vision, color=blindness, and night-blindness.
Short-sight, near-sight, or myopia (derived from the Greek words myo, I close, op, the eye), is often popularly confounded with dim or weak sight; but in reality short-sight applies exclusively to the range and not to the power of sight, and a short-sighted per son may possess the acutest power.of vision for near objects. In this affection the rays which ought to come to a focus upon the retina converge to a point more or less in front of it. The cause of this defect probably differs in different persons. It may arise from over-convexity of the cornea or the lens, from undue density or abundance of the humors of the eye, from elongation of the globe in its antero-posterior diameter. or from an imperfect,power of the eye to adjust itself to objects at various distances. The distance at which objects are perceived most distinctly by the perfectly normal eye ranges from 16 to 20 in.; an eye which cannot perceive objects distinctly beyond 10 in. may fairly be regarded as short-sighted; and in extreme cases the point of distinct vision may be 3, 2, or even only 1 in. from the eye. Short-sight is frequently hered itary in families. As a general rule the inhabitants of towns are Much more liable to it than persons living in the country, and students and literary men are the most liable of all. While in the foot-guards, consisting of nearly 10,000 men, " not half a dozen men have been discharged, nor have a dozen recruits been rejected on account of this imper fection, in a space of 20 years, in one college at Oxford no less than 32 short-sighted men (or m yopcs, as they are termed by some oculists) were met with out of 127" (Don ders, On the Accommodation and Refraction of the Eye, Load. 1864, p. 342). The fre quency of this affection in the cultivated ranks points directly to its principal cause— tension of the eyes for near objects. The myopia depending, as Donders believes, upon prolongation of the visual axis, this eminent physiologist inquires: "How is this pro longation to be explained? Three factors may here come under observation: 1. Prcs
sure of the muscles on the eye-ball in strong convergence of the visual axis; 2. Increased pressure of the fluids resulting from accumulation of blood in the eyes in the stooping position; 3. Congestive processes in the base of the eye, which, leading to softening, give rise to extension of the membranes. That in increased pressure the extension occurs principally at the posterior pole is explained by the want of support from the muscles of the eye at that part. Now, in connection with the causes mentioned, the injurious effect of fine work is, by imperfect illumination, still more increased; for thus it is rendered necessary that the work be brought closer to the eves, and that the stooping position of the head, particularly in reading and writing, is also increased. Hence it is that in schools where, by bad light, the pupils read bad print in the evening, or write with pale ink, the foundation of myopia is mainly laid. On the contrary, in watchmakers, although they sit the whole day with a magnifying-glass in one eye, we observe no development of myopic, undoubtedly because they fix their work only with one eye, and therefore converge but little, and because they usually avoid a very stoop ing position."—Op. cit. pp. 343, 344.
So far from short-sightedness improving in advanced life, as is popularly believed. it is too frequently a progressive affection: and every progressive myopia is threatening with respect to the future. " If," says Donders, " it continues progressive, the eye will soon, with troublesome symptoms, become less available, and not unfrequently, at the age of Z10 er 60, if not much earlier, the powerof vision is irrevocably lost, whether through separation of the retina from the choroid, from effusion of blood, or from atrophy and degeneration of the yellow spot." In the treatment of myopia the principal objects are: 1. To prevent its further (level opement and the occurrence of secondary disturbances; and 2. By means of suitable glasses, to render the use of the myoptic eye easier and safer.