80 THE POPULAR SCIENCE MONTHLY. 



numerical rank " from an ophthalmic point of view" to the different 

 schools examined by him. 



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

 he 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 un- 

 derstanding, 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 re- 

 flected or radiated from it enter the eye and produce a perfect picture 

 of the object upon the retina. But the perfection of the picture de- 

 pends 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 di- 

 minishes, rays entering the eye from any given point of the object be- 

 come more and more divergent. Now, unless there be a correspond- 

 ing 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 accommo- 

 dation, which encircles the lens. Thus, the point of convergence is 

 maintained upon the retina, in spite of the varying angle of the en- 

 tering 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. Conver- 

 gence 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 elon- 

 gated from front to back, the convergence will be at a point in front 



