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Disturbances Sight

vision, eye, condition, eyes, glasses and wearing

SIGHT, DISTURBANCES OF.—The power of vision may be affected by a great variety of disturbances, congenital as well as acquired. These may he caused by a diseased condition of any part of the eye, or they may result from deviations from the normal which cannot properly be spoken of as disease. Deviations from the normal condition of refraction must be included in the latter class. The normal eye is able to perceive clearly objects near at hand as well as those at some distance, the curvature of the lens adjusting itself according to the range of vision required. At about the age of forty-five, however, far-sightedness (presbyopia) begins to set in, the cili ary muscle gradually losing its power to contract and to increase the curv ature of the lens for near vision (see p. 162). This condition can be remedied by wearing convex eye glasses, the strength of which must be increased with advancing years.

Long-sightedness pia) is a somewhat similar condi tion, in which the parallel light-rays from a somewhat distant point are focussed behind the retina. The eye, therefore, has to accommodate for such rays, which subjects it to a constant strain that can be overcome only by wearing convex glasses. Long-sightedness is a con genital affection, and is frequently inherited. If nothing be done to counteract the defect, it may gradually give rise to annoying manifestations, such as headache and pain in the eyes.

Near-sightedness (myopia) does not imply weakness of sight, the dis turbance appertaining only to the range of vision, not to its acuteness. This condition occurs when the image of the object viewed forms in front of the retina. It may be due to various causes, the most frequent being elongation of the anteroposterior axis of the eye. The condition may be brought about by overstrain of the eyes, as by reading fine print in a dim light. The treatment of myopia includes the wearing of concave glasses ; and the eyes should receive constant medical attention in order to avoid possible internal changes.

The term " astigmatism " refers to a defect of vision caused by an irregular curvature of the surface of the cornea. The defect may he congenital, or it

may arise in consequence of diseases of the eye, particularly inflammation of the cornea. It is frequently an accompanying feature of near-sightedness, and is best corrected by wearing cylindrically ground glasses.

Asthenopia, or weakness of vision, consists in an inability to use•the eyes for any length of time without experiencing disagreeable symptoms, such as headache, pain about the eves, the shedding of tears, vertigo, and even nausea and vomiting. The affection may arise as a symptom of some severe nervous disorder ; or it may be due to a weakened condition of one or more of the muscles moving the eyeball, especially the ciliary muscle, the contractions of which adjust the curvature of the lens for vision at various distances.

Squinting (strabismus) may give rise to severe disturbances of sight by producing double vision. See SQUINTING.

Nystagmus is a condition characterised by twitching of the eyelids or by spasmodic movements of the eyeball, either rotary or from side to side. It is usually the result of working in dim light and in a stooping posture, so that the gaze is directed obliquely upward. The affection frequently occurs in miners. Nystagmus may also be the forerunner of a severe nervous trouble.

The choice of eye-glasses should never he left to the optician, for only a physician is able to make a 'correct examination of vision, by viewing the interior of the eye with the aid of an eve-mirror ; and upon this examination depends the choice of suitable glasses. Upon the first evidence of weakening eyesight, a physician should be consulted without delay. Properly selected eye-glasses can do no harm under any circumstances, but will more probably be beneficial.

For other affections of the eyes, see AMAUROSIS ; AMBLYOPIA ; BLIND NESS ; COLOUR-BLINDNESS ; EYE, DISEASE OF ; GLAUCOMA ; HEMERALOPIA ; LACRIMAL GLAND, SAC, AND DUCT, DISEASES OF ; MUSC.E VOLITANTES ; NYCTALOPIA ; SCOTOMA ; SNOW-BLINDNESS ; SQUINTING.