VISION, Defects of. The conditions classed as defects of sight or vision are mainly due .to anomalies in the structure of the eye, causing errors of refraction and other visual irregularities. Among the more common of these defects is that of astigmatistn (q.v.), which is usually characterized by a symmetry m the curvature of the cornea (q.v.) in different meridians. Color-blindness (q.v.) is a serious disability in certain occupations. Day-blindness (nyctalopia) is a condition in which one sees better in a dim light than in a bright light, due to some opacity in the cornea, the crystalline lens, etc. It occurs in amblyopia (q.v.) and other affections which produce dimness of vision. Night-blindness (hemeralopia), on the other hand, is a state in which the person af fected has normal vision in an ordinary light, but in twilight becomes suddenly blind. Double vision (diplopia) occurs when, as in some cases of squinting, tach eye sees things separately. This defect arises from derangement of the visual axis, sometimes through muscular par alysis.
In longsightedness or farsightedness (hy peropia or hypermetropia) objects are seen dis tinctly only at a range beyond that belonging to normal vision. Owing to the shortness of the eye-cavity the lenses are unable to converge the rays to a focus within the limits of the eye chamber, the image being, therefore, formed (theoretically) behind the eye. This defect is corrected by the use of convex lenses, which by converging the rays of light cause the image to fall on the retina. Shortsightedness or near sightedness (myopia) is the reverse of long sightedness in causes as well as in effects. In this, owing to the too great power of the crys talline lens or to the extension of the eye-cavity too far bacicward images from objects at some distance are formed in front of the retina, mak ing the sight confused, if not entirely defective, for things beyond a certain limited range, while rendering it very clear for near objects. For remedying this condition biconcave glasks are employed, which, unless the myopia is serious, need be used only for looking at objects far off. Shortsightedness and longsightedness are usu ally congenital.
Presbyopia is a defect similar to hyperopia and usually comes on with advancing years, naturally beginning from the 40th to the 45th year. It is due to diminished focusing power and lessened elasticity of the lens, the result of which is that the image of a near object is not clearly formed on the retina, but is formed be hind it, while distant objects are seen as well as ever. Convex lenses are used for remedying
this condition.
Strabismus or squinting is a deformity often seen, and is ascribed to want of parallelism in the visual axes when the effort is made to direct both eyes to an object at the same time. It may be due to loss of power (paralysis) of one or more of the eye muscles; and this may depend on a merely local affection, or may be a symptom of serious brain disease. But in the majority of instances and in all ordinary cases no such condition is present. The squint is said to be converg-ent when the squinting eye is directed toward the nose, and divergent when it is directed toward the temple; the convergent is much the more common. Concomitant strabismus is a variety of which the atnount con tinues about the same in all positions of the fixation-point. When the direction of the eye or eyes is upward or downward the squinting is said to be vertical. Convergent sqttint usually comes on during childhood, most often frnm thr ,rrnnd to the seventh year. It is sometimes due to defective sight in the squint ing eye, from congenital abnormality, severe inflammation or injury; but very often no such condition is present. In a large proportion of cases it is accompanied by hypermetropia, and is due to the increased effort of accommodation required to see near objects, being associated with an increased .and disproportionate effort of convergence. In such cases, if suitable glasses can be worn as soon as the squint begins to show itself, it may be prevented from becoming permanent. In other cases the pres ence of a squint may be traced to worms, the irritation of teething, etc.; and it disappears when the cause is removed. Divergent squint is very rarely present without considerable de fect in the sight of the squinting eye, except where it is the result of over-correction of a convergent squint by operation. It is often associated with myopia as the other fortn is with hypermetropia. The surgical operation for the correction of a squint consists in. the division of the muscle whose excessive activity leads to the faulty position —in convergent strabismus the internal rectus, in divergent the external. It is often necessary to operate on both eyes in the same manner, even where the squint is monocular. In some cases it. is requisite in addition to shorten the opposing muscle. See ASTIGMATISM; EYE; OPHTHALMIA; SENSES; VISION.