SIGHT, DEFECTS OF. Under thin head will be comprehended short-sight, long-sight, double vision, and the defective perception of colours, or colour blindness.
llfyepia, or Near-sightedness (from Aims, "I shut," and $. "the eye," a short-sighted person being in the habit of winking, or half shutting his eyelids when he endeavours to see objects distinctly).
When the images of surrounding objects are brought to a focus in the eye before they reach the retina, such an eye is myopic ; when, on the contrary, their foci would fall behind the retina, it is presbyopic. Individuals thus affected see all objects indistinctly that are viewed at the ordinary distance of distinct vision ; therefore, to remedy this defect, they bring them within such a distance of the eye as will ensure their images being brought to an exact focus upon the retina. The point of distinct vision (that is, the distance from the eye at which objects are perceived most clearly) of a perfect eye averages from 15 to 20 inches : an eye which cannot discern objects distinctly beyond 10 inches may be considered myopic ; but persons affected with a high degree of myopia have their point of distinct vision as near as two or three inches, or even one inch, to the eye. To short-sighted persons all objects appear magnified; they prefer to read a small type, and see better through a pin-hole in a card than with the naked eye : on the same principle, when they endeavour to aeo any distant object dis tinctly, they almost clone their eyelids. The explanation of these phenomena is to be sought for in the condition of the eyes themselves; they are generally firmer than usual, their corneae are preternaturally convex, and their pupils large; hence by diminishing the aperture through which the light is admitted, all but the more direct rays are excluded, and the images on the retina will be more defined.
The cause of myopia is an over-refractive condition of the eye; either the cornea or the crystalline lens is too convex, or the humours of the eye generally are too dense or too abundant.
Treatment —Although it is said that short-sightedness rarely comes ou before puberty, our own observations lead us to believe that it is more frequent1T a congenital defect than is generally imagined. If,
however, an incipient case were brought under the notice of the prac titioner which could be shown to have followed too great an exercise of the eyes upon minute objects, the cure would prohably be found in abstaining entirely for a time from such occupations, refraining also from the use of convex glasses, and employing the eyes chiefly upon large and distant objects. But this defect is one that is so little thought of, and is ao easily remedied by the use of glasses, that a medical man is seldom called upon to attempt its cure ; the only plausible means that have been recommended with this view are, practising the eyes in reading at gradually increasing distances, and a renunciation of such pursuits as require the concentration of vision upon near objects. The manner in which concave glasses improve the vision of near-sighted person,to, is by causing a divergence of the rays of light before they enter the says, thus counteracting the over-refractive condition of that organ. The glasses that are most commonly used are double concaves, of equal concavity on each side ; they are num bered 1, 2, 3, 4, &e., beginning with the longest focus or shallowest concavity. Unfortunately there is no uniform standard adopted in the manufacture of these glasses, so that what one optician calls No. 1, another rates as No. 2, and so on ; it is therefore advisable that those wish to fit themselves with spectacles should try a series of them at an optician's shop, and they should be content with the lowest number with which they can see objects clearly across the street ; if it diminishes them much, or gives them a dazzling appearance, or if the eyes feel strained after looking through them for a short time, they are too concave. Spectacles are always preferable to a single eye glass ; and when the individual has met with a pair which suit him, they should not be heedlessly changed for any of deeper concavity. It is also advisable not to wear them constantly, but only on occasions when their assistance is absolutely required.