SIGHT. 495 



sensitive surface perpendicularly, and thus convey the impression 

 of their direction from above or below. 



But beside the sensibility of the retina, the perfection and value 

 of the sense of sight depend very much on the arrangement of the 

 accessory organs, the most important of which is the crystalline lens. 



The function of the crystalline lens is to produce distinct perception 

 of form and outline. For if the eye consisted merely of a sensitive 

 retina, covered with transparent integument, though the impressions 

 of light would be received by such a retina they could not give 

 any idea of the form of particular objects, but could only produce 

 the sensation of a confused luminosity. This condition is illus- 

 trated in Fig. 156, where the arrow, a, b, represents the luminous 

 object, and the vertical dotted line, at the right of the diagram, 

 represents the retina. Rays, of course, will diverge from every 

 point of the object in ever} 7 direction, and will thus reach every 

 part of the retina. The different parts of the retina, consequently, 

 1, 2, 3, 4, will each receive rays coming both from the point of the 

 arrow, a, and from its butt, b. There will therefore be no distinc- 

 tion, upon the retina, between the different parts of the object, and no 



Fig. 156. Fig. 157. 



definite perception of its outline. But if, between the object and the 

 retina, there be inserted a double convex refracting lens, with the 

 proper curvatures and density, as in Fig. 157, the effect will be dif- 

 ferent. For then all the rays emanating from a will be concentrated 

 at x, and all those emanating from b will be concentrated at y. 

 Thus the retina will receive the impression of the point of the 

 arrow separate from that of the butt ; and all parts of the object, 

 in like manner, will be distinctly and accurately perceived. 



This convergence of the rays of light is accomplished to a certain 

 extent by the other transparent and refracting parts of the eyeball ; 

 but the lens is the most important of all in this respect, owing to 

 its superior density and the double convexity of its figure. The 

 distinctness of vision, therefore, depends upon the action of the 



