524 THE NERVOUS SYSTEM. 



Helmholtz, parallel rays would be brought to a focus by the cornea 

 alone at a point 10 millimetres behind the retina. But on passing 

 through the lens, their convergence is increased to such a degree that 

 they are concentrated at the retina. 



The function of the crystalline lens is to give perception of form and 

 outline. If the eye consisted only of a sensitive retina, covered with 

 transparent integument, although impressions of light would be received 

 by such a retina, they could give no idea of the form of objects, but 

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

 trated in Fig. 131, where the arrow, a, b, represents the luminous object, 

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

 retina. The rays diverging from every point of the object will thus 

 reach every part of the retina (1, 2, 3, 4,) ; and each one of these parts 

 will receive rays coming both from the point of the arrow, a, and from 

 its butt, b. There will, therefore, be no distinction, upon the retina, 

 between different parts of the object, and no perception of its figure. 

 But if, between the object and the retina, there be inserted a lens, with 

 the proper curvatures and density, as in Fig. 132, the effect will be dif- 

 ferent. All the rays emanating from a will then be concentrated at x t 



FIG. 131. 



VISION WITHOUT A LENS. VISION WITH A LENS. 



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 its butt ; and all parts of the object, in like manner, will 

 be distinctly perceived. 



The action of a lens, in thus focussing luminous rays at a particular 

 point, may be illustrated in the following manner: If a sheet of white 

 paper be held at a short distance from a candle flame, in a room with no 

 other source of light, the whole of the paper will be moderately and 

 uniformly illuminated by the diverging rays. But if a double convex 

 glass lens, with suitable curvatures, be interposed between the paper 

 and the light, the outer portions of the paper will become darker and 

 its central portion brighter, because a portion of the rays are diverted 

 from their original course and bent inward. By varying the distance 

 of the lens from the paper, a point will at last be found, where none of 

 the light reaches the external parts of the sheet, but all of it is concen- 

 trated upon a single spot ; and at this spot will be seen a distinct image 

 of the end of the candle and its flame. 



Perception of the figure of external objects therefore depends on the 



