THE SENSES 787 



eye of man, by altering the curvature, and therefore the 

 refractive power of the lens. That the cornea is not alone 

 concerned in accommodation, as was at one time widely 

 held, is shown by the fact that under water the power of 

 accommodation is not wholly lost. Now, the refractive 

 index of the cornea being practically the same as that of 

 water, no changes of curvature in it could affect refraction 

 under these circumstances. That the sole effective change 

 is in the lens can be most easily and decisively shown by 

 studying the behaviour of the mirror images of a luminous 

 object reflected from the bounding surfaces of the various 

 refractive media when the degree of accommodation of the 

 eye is altered. Three images are clearly recognised : the 

 brightest an erect virtual image, from the anterior (convex) 

 surface of the cornea ; an erect virtual image, larger, but 

 less bright, from the anterior (convex) surface of the lens ; 

 and a small inverted real image from the (concave) posterior 

 boundary of the lens (Purkinje-Sanson images). The second 

 image is intermediate in position between the other two. 

 It is possible with special care to make out a fourth image ; 

 but since it is reflected from the posterior surface of the 

 cornea, at which only a slight change in the refractive index 

 occurs, it is less brilliant than the first three. When the 

 eye is accommodated for near vision, as in focussing the 

 ivory point of the phakoscope (Fig. 330), the corneal image is 

 entirely unchanged in size, brightness, and position. The 

 middle image diminishes in size, comes forward, and moves 

 nearer to the corneal image. This shows that the curvature of 

 the anterior surface of the lens has been increased that is to 

 say, its radius of curvature diminished for the size of the 

 image of an object reflected from a convex mirror varies 

 directly as the radius of curvature. A slight change takes 

 place in the image from the posterior surface of the lens, 

 indicating a small increase of its curvature too. By means 

 of a method founded on the observation of the changes in 

 these images, and a special instrument called an ophthalmo- 

 meter which allows of their measurement, Helmholtz has 

 calculated that, during maximum accommodation, the radius 

 of curvature of the anterior surface of the lens is only 6 mm., 



502 



