480 



THE HUMAN BODY. 



upon it; so, too, with the rays from B: and if the focal dis- 

 tance (see Physics) of the lens be properly adjusted these 

 points of convergence will both lie on the retina, that for 



FIG. 121. Diagram illustrating the indistinctness of vision with a retina alone. 

 K, a surface on which are two spots, A and B; r r, the retina. The diverging 

 lines represent rays of light spread uniformly over the retina from each spot. 



rays from A at a, and that for rays from B at ~b. The 

 sensitive surface would then only be excited at two limited 

 and 'separated points by the red light emanating from the 

 spots; consequently only some of its end organs and nerve- 



FIG. 122. Illustrating the use of a lens in giving definite retinal images. A, B, 

 K, r r, as in Fig. 121. L, a biconvex lens so placed that it brings to a focus on 

 the points a and 6 of the retina, rays of light diverging from A and B re- 

 spectively. 



fibres would be stimulated and the result would be the 

 recognition of two separate red objects. In our eyes are 

 found certain refracting media which lie in front of the 

 retina and take the place of the lens L in Fig 122. That 

 portion of physiology which treats of the physical action of 

 these media, or in other words of the eye as an optical in> 

 strument, is known as the dioptrics of the eye. 



The Appendages of the Eye. The eyeball itself con- 

 sists of the retina and refracting media, together with 

 supporting and nutritive structures and other accessory 



