THE EYE 341 



are not exactly alike. The blending of the images will 

 give the idea of solidity or of position in distinction from 

 the impression that everything is a flat surface. This per- 

 ception is not natural, but must be learned. Distant objects 

 always seem flat. 



621. Movements of the eyes. If the images of an object do 

 not fall upon corresponding parts of the retinas of the two eyes, two 

 images will be seen. Sometimes a muscle will draw one eye aside so 

 that it does not look in the same direction as the other eye. A person 

 with this defect is said to be cross-eyed. 



Young babies have no control of their eye muscles, and so have no 

 distinct vision, except as they catch accidental glimpses. A bright 

 cloth gives them a sensation of color at whatever distance it is held, 

 and so amuses them. At about the age of three months they begin to 

 gain control of their muscles, so that they can focus the eyes and turn 

 them to any object at will. It takes them several months more to 

 acquire a knowledge of solidity and position. 



Anything touching the eye causes the lids to close in a reflex man- 

 ner for protection. Tears flowing over the eye cause the lids to wink 

 and distribute the moisture over the whole surface. 



In reading in the cars, the constant jarring of the paper compels- the 

 muscles of the eyes to be in constant action to adjust the eye to the 

 ever-varying positions and distances. So they soon become tired and 

 ache. In reading while lying down, the eyes must look toward the 

 feet. The constant strain of turning the eyes down tires the muscles, 

 so that the vision becomes blurred. 



622. Diseases of the eye. An ulcer or scar upon the cornea, 

 closure of the pupil, cataract, and wasting of the optic nerve are com- 

 mon causes of blindness. If your eyes are sore, or red, or painful, 

 you should consult a doctor at once; for blindness may result from 

 what seems to be a mild trouble. 



A particle of sand or other substance between the eyeball and the 

 lids causes great pain. Rubbing the eyelid forces the particle into the 

 delicate flesh and increases the pain and danger. If the lid is gently 

 held away from the eyeball for a moment, the tears may wash out the 

 particle. 



The eye is wel! protected from injury, for the sclerotic coat is the 

 strongest tissue ic the body. 



