ABNORMALITIES IN THE REFRACTION OF THE EYE 



861 



convergent rays of light, but since there are no convergent rays or- 

 dinarily available in space, this direction must first be imparted 

 to the parallel and divergent rays by means of a convex lens (Fig. 

 460). The converging power of the lens interposed in front of the 

 eye, must be proportional to the degree of the hypermetropia. 



Keeping these facts clearly in mind, it will be seen that the condi- 

 tion of presbyopia developed in later years, must improve the vision 

 of the myopic person, but diminish that of the hypermetropic. Ob- 

 viously, the gradual flattening of the lens in consequence of the effects 

 of old age reduces its refractive power and forces the focal point 

 farther backward. If the eye is hypermetropic, the presbyopia 

 makes matters worse, because it tends to move the focal point still 

 farther "behind" the retina. In the myopic eye, on the other hand, 

 a distinct improvement must result, because the presbyopic lens does 

 not converge the rays so strongly, and hence, permits their focal 



FIG. 460. DIAGRAM TO ILLUSTRATE THE REFRACTION IN A HYPERMETROPIC EYE. 

 L, Luminous point focalized in L l "behind" the retina. A convex lens C renders 

 these rays more convergent so they are made to intersect upon the retina in L 2 . 



point to move closer to the retina. Conversely, a presbyopic eye may 

 be greatly benefited by the subsequent development of a myopia, 

 because the recession of the focal point is then compensated for by a 

 displacement of the retina in a backward direction. These phenomena 

 are generally designated as "second sight." 



To summarize: An emmetropic eye (Fig. 461, E] brings parallel and 

 even divergent rays of light to a sharp focus upon the retina, while 

 a myopic eye (M) focalizes them in. front and a hypermetropic eye, 

 (H) "behind" the retina. In order to render M emmetropic, the 

 entering rays of light must be diverged by means of a concave lens, 

 while H can only be made emmetropic by converging them with the 

 aid of a convex lens. 



Simple Methods Used to Determine the Refractive Power of the 

 Eye. The acuity of vision may be tested in different rays. Snel- 

 len's test types consist of a series of letters placed at a distance of 5 m. 

 from the eye. It has been determined that the smallest object which 

 a normal eye is capable of distinguishing at this distance, measures 



