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This eye defect, which occurs in about twenty pei cent. of the population, is due to a lengthened eyeball, which prevents a clear focus on the retina. The condition can only be corrected by the wearing of glasses.
Popularly known as short sight, the term myopia is derived from a Greek word meaning to shut the eyes, and was given to the condition on account of the habit possessed by myopes of screwing up the eyelids in order to see distant objects more clearly. In the perfectly normal or emmetropic eye at rest, parallel rays of light, i.e. rays coming from a light source at an infinite distance, come to a focus on the retina and form a clear image there. Actually, objects at a distance greater than six yards form a sharp image on the retina of the normal resting eye. The myopic eye is longer from before backwards than the normal, and parallel rays entering it are brought to a focus by the lens system of the eye before reaching its retina, as shown in the diagram on this next page. They then diverge again and form only a blurred image on the retina. Only rays which are diverging from some point nearer to the eye will come to a focus on the retina of the myopic eye, because such rays are not brought to a focus as quickly as parallel rays.
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The myope therefore sees near objects clearly but is unable to see distant objects well. He misses entirely the details of landscape, while myopic school children cannot see what is written on the blackboard from the back of the class.
The eye has no device for correcting this error, and a glass has to be found which will cause rays from distant objects to diverge before entering the eye, as if they came from a point nearer to the eye. A concave or diverging lens will do this, and the diagram shows how a lens of this type and of the right strength causes parallel rays to form a sharp image on the retina of the myopic eye.
Persons affected with myopia, then, in order to see distant objects clearly, must wear concave lenses.
It is especially important that shortsighted children should wear glasses constantly, and that they should be reexamined by a doctor about every twelve nonths during the growing period, because it as during thas period that myopia tends to increase.
Children are generally born hypermetropic and gradually develop an eyeball of normal shape, so that by the age of ten they are emmetropic. In certain cases, however, the increase in the size of the eye continues and the long eyeball of myopia is the result, one millimetre increase in length of the eye giving a myopia of such a degree that the farthest point of distinct vision is little more than a foot away from the eye.
Myopia occurs in twenty per cent. of the population, but in most cases it does not increase after twenty years of age. The popular notion that short sight is strong sight is a fallacy, probably arising out of the fact that myopes are able to read very fine print held close to their eyes and that some do not require reading glasses in later life.
Care of Myopic School Children
The development of myopia occurs, then, chiefly during the usual period of school life, and although it is not proved that school work is responsible for it, it is certain that the excessive use of the eyes for too fine work and the bad position frequently adopted by children over their lessons have a marked deleterious effect on the eye. For this reason great care should be taken in the choice of print for school books, and children should be encouraged to form good habits with regard to posture at the school desk.
Rays of light coming from a distance, that is, over 18 feet or more, are approximately parallel. In the normal eye, A, these are brought to a focus on the retina without any effort of accommodation, but in the myopic eye, B, because it is unduly long, they come to a focus in front of the retina. From a nearer point, however, they focus on the retina (C) The remedy for myopia is to place a concave lens in front of the eye, D, when the distant rays are focussed as if they originated as in C.