280 PHYSIOLOGY FOR DENTAL STUDENTS. 



focused in turn on the retina. These movements are effected 

 by the so-called ocular muscles. 



There are therefore three functions involved in the act of 

 seeing : (1) That of the retina, in reacting to light. (2) That of 

 the cornea, etc., in focusing the light. (3) That of the ocular 

 muscles, in moving the eyeball. 



The Optical Apparatus of the Eye. 



It will readily be seen that the eye is constructed on much 

 the same principle as a photographic camera, the retina bring 

 like the sensitive plate. There is, however, an important dif- 

 ference in the manner by which objects at varying distances are 

 brought to a focus on the sensitive surface in these two cases: 

 in the camera, it is done by adjusting the distance between the 

 lens and the focusing screen; in the eye, it is done by varying 

 the convexity of the lens. 



In order to understand how the optical apparatus works, it 

 is necessary to know something about the refraction of light. 

 When a ray of light passes from one medium to another, it be- 

 comes bent or refracted. When it passes from air to water or 

 glass, for example, it becomes refracted so that the angle which 

 the refracted ray makes with the perpendicular to the surface 

 is less than that of the entering ray. In other words, the ray 

 becomes bent towards the perpendicular. The greater the dif- 

 ference in density between the two media, the greater is the 

 difference between the two angles. A figure expressing the ratio 

 between these two angles is called the index of refraction. If 

 the ray of light leaves the denser medium by a surface which 

 is parallel with that by which it entered (as in passing through 

 a pane of glass), it will be refracted back to its old direction. 

 but if, as in a prism, it leaves the denser medium by a surface 

 which forms an angle with that by which it entered, the original 

 refraction will be exaggerated. If two prisms be placed with 

 their broad ends together, parallel rays of light coming from a 

 certain direction will be bent so that, on leaving the prisms, 

 they meet somewhere behind them. Two prisms so arranged are 

 virtually the same as a biconvex lens. It is plain that the 





