THE HUMAN EYE 



283 



actors in a very slightly different position and so the 

 observer gets the impression that the actors are actually 

 moving in the picture. Sometimes thirty pictures are 

 thrown on the screen per second; this gives a better 

 effect than sixteen per second. 



194. Defects of the Eye. There are two common de- 

 fects of the eye caused by the eyeball not having the 

 proper shape and by the inability of the crystalline lens 

 to adjust itself to various dis- 

 tances. Such eyes lack normal 

 accommodation. The remedy is 

 to wear glasses with lenses of the 

 proper shape to make up for the 

 defect of the eye. 



(a) Nearsightedness. This is 

 most often due to the eye being 

 too long from front to back. The 

 rays of light are brought to a 

 focus before reaching the retina Showing the defect in A and 



which makes the object appear the correction with a concave 



blurred or else it is not visible at 



all. Sometimes nearsightedness is the result of the 

 cornea or the crystalline lens being too convex. This is 

 usually caused by a weakening of the eyeball and does 



NEARSIGHTED EYES 



A 



FARSIGHTED EYES 

 Showing the defect in A and the correction with a convex lens in B. 



not occur often in very young children. The remedy for 

 nearsightedness is to wear glasses with diverging lenses. 



