FARSIGHTEDNESS AND NEARSIGHTEDNESS 123 



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 

 adjustment of the muscles is so quick and unconscious that 

 we normally 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 most nearly perfect in the 

 young. 



117. Farsightedness and Nearsightedness. A farsighted 

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

 objects, and in many cases a 

 farsighted person cannot see 

 near objects at all. The crys- 

 talline lens is so thin in the 

 center that even when the 

 muscles are doing their best to 

 bulge the lens, it is still not 

 bulged enough (Fig. 80) to focus near objects upon the retina. 

 Since the crystalline lens is too thin at the center, the defect 

 may be remedied by wearing convex lenses which are thick 

 enough in the center to 'balance the too great thinness of the 

 crystalline lens. 



FlG. 79. The normal eye. 



FlG. 80. The farsighted eye. The defect is remedied by convex glasses. 



A nearsighted person is one who cannot see objects dis- 

 tinctly unless they are very close to the eye. In this case the 



