Short and Long Sight.
Short Sight, called myopia, depends on too great a convexity of the cornea, or crystaline lens, or vitreous humor, one or all, and the consequent formation of the image of the object inspected a little in front of the optic nerve, or retina, as at a (Fig. 183), instead of at b, where it should be formed. The rays of light are brought to a focus before they reach the retina.
Children are either born with this defect, or it is brought on by too close study, or by long application of the eyes to minute objects.
It may be remedied frequently by exercising the eyes in looking at distant objects. Children afflicted in this way should have their studies abridged, and their exercise in the open air increased. While studying they should have some apparatus applied to them which shall keep the chin elevated, so that the head cannot be dropped too low, and the eyes brought too near the book. And the book should each day be placed a very little further from the eyes.
Glasses worn by persons having this defect of vision should be concave, as at c.
Long Sight, or presbyopia, depends on the humors of the eye not being convex enough. In this case, the image of the inspected object is formed beyond the optic nerve, as at d (Fig. 184). This is one of the earliest signs of advancing age.
This defect is to be remedied by glasses which are convex, . Persons in the early autumn of life must not resort to glasses too hastily, or, indeed, until they are compelled to, nor should they change those first used too soon. Glasses should make objects look distinct and bright, but not larger than natural.
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