156 HUMAN PHYSIOLOGY 



normal eyes who has reached the age of forty-five usually 

 finds that he cannot focus small type. He must begin 

 to use convex glasses for reading. It should be pointed 

 out that when he puts on his glasses he is adding to the 

 total refracting power of his optic system and the prin- 

 ciple of accommodation extra convexity is still utilized. 

 The supplementary lens is placed in front of the eye when 

 the lens inside can no longer be rounded up to meet the 

 requirement. The loss of the capacity for accommodation 

 which comes with advancing age is called presbyopia. 



Defects of Vision. A person who is near-sighted has 

 an eyeball which is deeper than normal. The result is 

 the same that can be demonstrated with a camera: 

 if the distance between the lens and the plate is too 

 great for general work there will be sharp images of near 

 objects but a blurred background. The act of accommo- 

 dation makes a near-sighted eye worse than when at 

 rest. Accordingly, the near-sighted have little use for 

 accommodation. In old age they may be very proud of 

 the fact that they can read without glasses. 



Glasses to correct near-sight must be concave. Their 

 effect is to postpone the meeting of the rays which are 

 to be focused until the retina has been reached. Such 

 glasses have a function just contrary to that of accommo- 

 dation. They sacrifice near vision in favor of distant. 

 Provided with them, the near-sighted person has normal 

 vision for distance and uses his accommodation power for 

 near work as the normal subject would do. Some- 

 times a condition resembling near-sight may exist for a 

 time and presently correct itself. This is due to a 

 persistent contraction of the accommodation muscle 

 which the victim cannot inhibit. 



The term far-sight is often used to indicate superior 

 visual power. But it is better reserved to indicate an 

 abnormality. The trouble here is that the eyeball is 

 too shallow. When the nervous mechanism is at rest 

 nothing is strictly in focus for the far-sighted eye. The 

 facts can be verified by closing the bellows of a camera 



