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HUMAN BIOLOGY 



in a different way, namely, by making the elastic lens more or less 

 convex. 



246. Sensations of sight. We shall now try to see how it is that 

 the eye helps us to get sensations of sight. If an object, say an 

 arrow, is held in front of the eye, rays of light pass in a great many 

 directions from every part of the arrow tip. A considerable number 

 of these rays strike the convex surface of the cornea and the crystal- 

 line lens, and are thereby focused, or made to converge upon a point 

 on the retina. In the same way the light rays from every other part 

 of the arrow are brought to focus on the inner surface of the retina. 

 By this means a smaller, inverted image, of the arrow (Fig. 53) is 



FIG. 53. Formation of an image on the retina. 



projected on the inner lining of the eye. The influence of these light 

 rays then passes through the layers of the retina, and when these 

 so-called " messages " traverse the nerve fibers and reach the brain, 

 we become conscious of sensations of sight. 



247. Defective eyes. A normal, healthy eye has the power of 

 adjusting itself so that objects become visible which are within five 

 to ten inches, or as far away as a distant horizon. Many people, 

 however, find that they can see objects near at hand much 

 more clearly than those at a distance ; in other words, they are 

 nearsighted. Others, on the other hard, are farsighted. These 

 defects in vision are due to imperfect formation of the eye, and can 

 be corrected only by the use of proper eyeglasses or spectacles, which 

 should be purchased only on the recommendation of a competent eye 

 specialist. 



Another very common defect of the eye is known as a-stig'- 

 ma-tism. Many people, on looking with each eye separately at 



