SIGHT (AS. ge-sihP, OHG. ye-situ, Ger. Cc sicht, from AS. sFon, OHG. sehan, C;er. sehen, to see; connected with Lat. sequi, Gk. ercaOat, hepes thai,Lith. sckti, Skt. sac, to follow), DEFECTS OF. Under this head we shall consider certain af fections of the eyesight due to some known or unknown peculiarity of the optical apparatus (including the optic nerve)—viz. near-sighted ness, far-sightedness. double vision, color-blind ness, night-blindness, and day-blindness. Defects due to errors of refraction include the first two of these.
Near-sightedness, short-sightedness, or myopia 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 person 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 prob ably differs in different persons. It nearly al ways arises from elongation of the globe in its antero-posterior diameter, more rarely from in creased curvature of the cornea, increase in re fractive power of the lens in the early stage of senile cataract, or from an imperfect power of the eye to adjust itself to objects at various dis tances. The distance at which objects are per ceived most distinctly by the perfectly normal eye ranges from 16 to 20 inches: an eye which cannot perceive objects distinctly beyond 10 inches may fairly he regarded as short-sighted; and in extreme cases the point of distinct vision may be 3, 2, or even only 1 inch from the eye. There is frequently an hereditary ten dency to near-sightedness, but it is rarely con genital. It is often acquired by excessive use of the eyes at an early age for reading or other near work. Overstudy under unfavor able circumstances and poor health favor its development. As a general rule the tants 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. The fre quency of this affection in the cultivated ranks points directly to its principal cause—tension of the eyes for near objects. Prolongation of the
visual axis is attributed to (1) pressure of the muscles on the eyeball 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 mem branes; (4) the shape of the orbit in broad faces, causing excessive convergence, the trouble occur ring especially in such persons. That in increased pressure the extension occurs principally at the posterior pole is explained by the want of sup port 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 eyes, and that the stooping position of the head, particularly in reading and writing, is also increased. Bence it is that in schools where, by bad light, the pupils read bad print in the evening, or write with pale ink, the founda tion 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 myopia, undoubtedly because they fix their work only with one eye, and, therefore, converge but little, and because they usually avoid a very stooping position.
So far from short-sightedness improving in ad vanced life, as is popularly believed, it is too fre quently a progressive affection: and every pro gressive myopia is threatening with respect to the future. Those eases in which the myopia de velops slightly in young persons and practically becomes arrested are called simple or stationary myopia. Progressive myopia is the form which increases steadily with degenerative changes in the cho•oid and other deep structures. Persons with uncorrected myopia of any severity have a characteristic vacant expression from constant inability to see any other than near objects.