Page:Popular Science Monthly Volume 12.djvu/90

80 numerical rank "from an ophthalmic point of view" to the different schools examined by him.

Here arise two questions: 1. Can near-sight be cured? 2. Can it be prevented?

All authorities agree that it is incurable, and all agree that it may be prevented.

How?

The answer to this may be made more satisfactory if first we rapidly sketch a few well-known physiological facts, and get an understanding, approximately correct at least, of what near-sight is, and what causes it. Incidentally we shall have occasion to notice some of the methods and appliances for detecting both near-sight and over-sight.

When we see any object clearly, it is because the rays of light reflected or radiated from it enter the eye and produce a perfect picture of the object upon the retina. But the perfection of the picture depends upon the distance, size, and illumination of the object relatively to the powers and condition of the eye. The distance determines the angle at which the rays enter the eye. Whatever this angle, the rays must converge upon the retina, or the picture will be defective. This convergence it is the office of the lens to effect. From remote objects the rays are parallel, or nearly so. These, passing through the lens, are converged by it upon the retina. As the distance diminishes, rays entering the eye from any given point of the object become more and more divergent. Now, unless there be a corresponding increase in the convexity of the lens, these divergent rays will not be focalized at the same point as were the parallel rays; because, with the same power of lens, the focal distance must increase as the rays diverge; they will not, therefore, have converged when they reach the retina. A perfect picture will not be formed, and distinct vision will not be realized. But a change does take place in the lens corresponding to the change in the angle of the rays which enter the eye. As they diverge, its convexity increases. This is effected by the contraction of a muscle called, sometimes, the muscle of accommodation, which encircles the lens. Thus, the point of convergence is maintained upon the retina, in spite of the varying angle of the entering rays.

The normal location of the retina is that point at which parallel rays are converged, the lens being at rest. But if the eyeball loses its normal shape, and becomes elongated in the direction of its visual axis, the retina is thereby set back beyond the focal point. Convergence may be effected within the normal distance, but never beyond it; for, while the lens may become changed from its passive state to one of greater convexity, it cannot assume a convexity less than that of its passive state. Consequently, when the eyeball becomes elongated from front to back, the convergence will be at a point in front