

FARSIGHTEDNESS AND NEARSIGHTEDNESS 259 



located directly back of the iris. How, then, does it happen 

 that we can see clearly both near and distant objects ; that the 

 printed page which is held in the hand is visible at one second, 

 and that the church spire on the distant horizon is visible the 

 instant the eyes are raised from the book ? How is it possible 

 to obtain on an immovable screen by means of a simple lens 

 two distinct images of objects at widely varying distances ? 



The answer to these questions is that the crystalline lens 

 changes shape according to need. The lens is attached to 

 the eye by means of small muscles, m, and it is by the action 

 of these muscles that the lens is able to become small and thick, 

 or large and thin ; that is, to become more or less curved. 

 When we look at near objects, the muscles act in such a way 

 that the lens bulges out, and becomes thick in the middle and 

 of the right curvature to focus the near object upon the screen. 

 When we look at an object several hundred feet away, 

 the muscles change their pull on the lens and flatten it until 

 it is of the proper curvature for the new distance. The adjust- 

 ment of the muscles is so quick and unconscious that we nor- 

 mally do not experience any difficulty in changing our range of 

 view from the object at our feet to the far-distant hills and stars. 



The ability of the eye to adjust itself to varying distances 

 is called accommodation. The power of adjustment in gen- 

 eral decreases with age, being nearly perfect in the young. 



Farsightedness and nearsightedness. A farsighted per- 

 son is one who cannot see near objects so distinctly as far 

 objects, and who in many cases cannot see near objects at all. 

 The eyeball of a farsighted person is very short, and the retina 

 is too close to the crystalline lens. Near objects are brought 

 to a focus behind the retina instead of on it, and hence are 

 not visible. Even though the muscles of accommodation do 

 their best to bulge and thicken the lens, the rays of light are 

 not bent sufficiently to focus sharply on the retina. In conse- 



CLARK INTRO. TO SC. IJ 



