352 



ADVANCED PHYSIOLOGY 



longed period that one has been looking at the book held neai 

 the eyes. 



Figures 175 and 176 show what the result would be if the 

 lens of the eye could not be accommodated to different dis- 

 tances. There would be 

 only one point at which 

 objects could be seen 

 clearly. If parallel rays 

 of light are focused on 

 the retina as in Figure 

 175, rays from points 

 near by, as at A, will 

 be focused behind it. 

 If, however, rays from 

 an object near the eye are 

 properly focused, as in 



Figure 176, objects farther away would be indistinct because 

 light from them would come to a focus in front of the retina. 

 The power of changing the shape of the lens allows objects 



at any distance (not too 

 near) to be seen clearly. 



Near- and Farsightedness. 

 The normal eye is of such 

 shape that parallel rays of 

 light are focused exactly on 

 the retina; Fig. 175. Often, 

 however, a person's eyeballs 

 are a little too long or the 

 lenses a little too convex 



FIG. 175. DIAGRAM 



Showing that when parallel rays of light are 

 focused on the retina, rays from near objects 

 will be focused back of the retina. 



FIG. 176. DIAGRAM 



Showing that when near objects are 

 focused on the retina, parallel rays of 

 light will be focused in front of it. 



ing rays of light coming from 

 on the retina but in front of 



(Fig. 176), so 

 a distance are 

 it, and to be 



that enter- 

 not focused 

 focused ex- 



actly on the retina, an object must be held close to the eyes. 

 Nearsightedness, or myopia, may result from either of these 

 causes, and in either case the difficulty may be remedied by 



