MYOPIA, HYPEROPIA 339 



vitreous body. Should any one of these media lose its transparency, 

 vision would be impaired or perhaps lost. 



The correct retinal image is the object for which these struc- 

 tures are designed. In order that this may be formed, the rays of 

 light which are reflected from a wide area of surrounding objects 

 must be made to converge and meet on the retina, from which 

 the stimulus thus received is conveyed to the brain by the optic 

 nerve. Rays of reflected light go toward the eye from every 

 direction and by concentrating those which enter the pupil upon 

 the retina, a small inverted image is formed which is recognized 

 by the brain as representing the objects so pictured in their proper 

 size and position. 



This concentration of rays of light is accomplished by the 

 refractive (or bending) media of the eye: the cornea, aqueous humor, 



FIG. 217. MYOPIA. FIG. 218 CORRECTION or MYOPIA BY 



Parallel rays focus at F, cross and A CONCAVE LENS. (Brubaker.) 



form diffusion-circles; divergent rays 

 from A focus on the retina. (Bru- 

 baker.) 



crystalline lens and vitreous body, in order. Each medium refracts 

 (or bends) the rays more and more toward a common center or 

 focus. The denser the media or the more convex the surface, 

 through which the light rays pass, the greater the change in their 

 direction (the shorter the focus). 



In the normal or emmetropic eye the focus is at the retina and 

 a clear image is formed. In the myopic or "near-sighted" eye it 

 falls in front of the retina, either because some surface (cornea or 

 lens or both) is too convex or the eye is too long, and the rays of 

 light from all except near objects, converge in front of the retina. 

 The remedy is a concave lens of glass to counterbalance the 

 excessive convexity or length. (See Figs. 217 and 218.) 



In the hyperopic or "far-sighted" eye, the focus would fall 

 behind the retina; the surface of the cornea or lens is not convex 

 enough or else the eye is too short. The rays of light are not 

 sufficiently bent to meet upon the retina and the remedy is a 



