THE EYE AND THE SENSE OF SIGHT 117 



a shortsightedness," or "longsightedness." In short- 

 sighted persons the rays which in normal eyes come to a 

 focus exactly upon the retina meet at a point in front of 

 it, so that no distinct image is formed. The eye is too 

 long from front to back, and the difficulty must be cor- 

 rected by using lenses which will carry the focus back to 

 the surface of the retina (Fig. 70). 



Fig. 70. Diagram showing position of retina. 

 In natural sight (5). In far sight (). In near sight (C7). 



Those who habitually use their eyes for seeing only 

 objects near at hand are apt to become shortsighted. It 

 has been found that children who have grown up in 

 crowded parts of smoky London, with little opportunity 

 for looking off long distances, are very often nearsighted. 

 On the other hand, sailors and others accustomed to use 

 the eye constantly for distant vision grow longsighted. 

 Their eyes become shortened from front to back, so that 

 the focus for ordinary vision is beyond the retina. 



As persons grow old the eye usually becomes flattened 

 on this axis, and glasses are needed to converge the rays 

 of light more rapidly. 



160. Squinting, etc. The muscles of the two eyes act 

 simultaneously so that the visual images are formed upon 

 corresponding parts of the two retinas. But if the internal 

 rectus muscle of one eye is paralyzed, or for some reason 



