Nervous System and Sense Organs 219 



In its action, the eye is very similar to a camera. The amount of 

 light entering is controlled by the diaphragm of the camera, and the 

 focusing is done by the lens on to the sensitive film. One fundamental 

 difiference ocurs here, however; in the camera, focusing is accomplished 

 by varying the distance from the lens to the film. For near objects, 

 the lens is moved away from the film; for far objects, it is moved closer. 

 The human eye accomplishes this same result by varying the shape of 

 the lens. Depth of focus or perception is attained in a similar manner 

 for both : the opening of the diaphragm or iris is controlled. Greater 

 depth for near objects is attained by limiting the amount of light ad- 

 mitted; for far objects, such control is not necessary. 



In spite of the efficiency of our method of accommodation, it is 

 not unreasonable to assume that equal efficiency could have been at- 

 tained by varying the position of the lens. And, in fact, this method 

 is found in some animals. In amphibians, the lens moves forward to 

 adjust for near objects; in most fish, the lens moves backward to adjust 

 for far objects. 



Depth perception in human beings is due to the position of the 

 eyes and the fact that two separate, overlapping images are perceived. 

 This binocular vision also is of great aid in judging distances. In 

 animals which have their eyes at a distance from one another, two 

 images entirely separated must be perceived. 



Many of the common difficulties with vision are due to struc- 

 tural defects of the eye. With increasing age, the accommodating ability 

 of the lens and ciliary body decreases. The eyes of young children, 

 for example, can focus from two inches to infinity ; those of an adult 

 from eight inches to infinity ; while in old age near vision is often im- 

 paired due to the hardening of the crystalline lens and the resulting 

 loss of elasticity. Farsightedness is thus a concomitant of old age. 



In the farsighted person, the light rays after passing through 

 the lens meet in a position slightly behind the retina; in the near- 

 sighted person, the image focuses slightly in front of the retina. 

 Both of these conditions can be corrected by proper lenses, a convex 

 lens for the farsighted eye and a concave one for the nearsighted eye. 



An even commoner defect than these is that of astigmatism in 

 which the refractive power of the lens or cornea is not the same. In 

 other words, the curvature of either or both of these two surfaces is 

 such that the light rays entering the eye are deflected unequally. Some 

 focus in front of the retina, others behind. When a person with 

 this condition looks at parallel radiating lines, one portion will be in 



