FAR AND NEAR SIGHT. 



309 



a near object the lens becomes thicker, and when we look at a 

 distant object the lens becomes less thick. In looking at an 

 object that is near, the ciliary muscle pulls on. the hyaloid 

 membrane, and draws it forward (since the muscle is fastened 

 at the point where the iris joins the cornea). When the 

 hyaloid membrane is pulled forward, the lens is released from 

 pressure that was given it by the lens capsule. Now the lens 

 becomes thicker because it is elastic, and when it is not sub- 

 ject to pressure it tends to become relatively thick. When 

 we look at a distant object the muscle relaxes, and the cap- 

 sule presses on the front of the lens and flattens it, so ad- 



CILIARY MUSCLE 



FAR NEAR CILIARY PROCESS 



Fig. 92. A Diagram to Illustrate Accommodation. 



justing for far sight. It should be noted that adjustment 

 for near sight is brought about by muscular effort, hence is 

 fatiguing; whereas adjustment for far sight is accomplished 

 mechanically, without effort. In old age the lens usually be- 

 comes less elastic, and cannot adjust for near sight. Since it 

 is unable to grow more convex, artificial lenses (eyeglasses) 

 may be used to enable one to see near objects clearly. Most 

 elderly people see fairly well at a distance, but use glasses for 

 reading or any close work. 



In " near-sighted " eyes, the eyeball is often too long from 

 front to back, so the image is not thrown on the retina. Con- 

 cave glasses remedy this defect. The eye may also be too 



