614 THE SENSES. 



radius of curvature for the anterior surface is, on the average, 10 milli- 

 metres, that for the posterior surface 6 millimetres. 



This makes the crystalline lens the most powerfully refracting body in 

 the eyeball, and by it said parallel or diverging luminous rays, after pass- 

 ing through the pupil, are brought to a focus at the situation of the 

 retina. This effect is not due entirely to the lens, since the convex form 

 of the cornea and the more or less spheroidal figure of the whole eye- 

 ball necessarily have in some degree a similar action upon rays enter- 

 ing from the front. According to Helmholtz, parallel rays would be 

 brought to a focus "by the cornea alone, if they were sufficiently pro- 

 longed, at a point situated 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 situation of the retina itself. 



The function of the crystalline lens is to produce distinct perception 

 of form and outline. If the eye consisted merely of a sensitive retina, 

 covered with transparent integument, although the impressions of light 

 would be received by such a retina, they could give no idea of the form 

 of particular objects, but would only produce the sensation of a confused 

 luminosity. This condition is illustrated in Fig. 190, where the arrow, 

 a, 6, 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 in every direction, 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 distinction, upon the retina, 

 between the different parts of the object, and no definite perception of 

 its figure. 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. 191, the effect will be different. All the rays emanating from 



Fig. 190. Fig. 191. 



VISION WITHOUT A LENS. VISION WITH A LENS. 



a will then be concentrated at a?, 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 and accurately perceived. 



The action of a refractive body with convex surfaces, in thus focussing 

 luminous rays at a particular point, may be readily illustrated in the 

 following manner. If a sheet of white paper be held at a short distance 

 from a candle flame, in a room where there is no other source of light, 



