IMAGES UPON THE RETINA 513 



of the lens 50.6 mm. The refracting power of the cornea is therefore 43.2 

 diopters 1 (1,000-^23.3), that of the entire eye 64.5 diopters. 



It follows that the strongest refraction of light takes place in the cornea. 



Occasionally among old people the lens becomes turbid and opaque. In 

 order to restore the sight in such cases the lens is removed. After the operation 



FIG. 210. Position of the cardinal points in the schematic eye, after Helmholtz. 



the refracting power of the eye, which can no longer be accommodated, is about 

 10 D. less than before i. e., in the normal eye of old persons the lens raises the 

 refracting power of the eye by about this amount. 



2. IMAGES UPON THE RETINA 



The size and position of an image formed by a centered optical system 

 depends not only upon the size and position of the object, but also upon the 

 position of the cardinal points of the system. From Fig. 209 it is evident 

 that so long as the distance e P > F E the image will be inverted i. e., so 

 long as an object is beyond the outer or anterior focal point of the eye the 

 image on the retina will be inverted. Again, so long as e P > 2 F E the image 

 will be smaller than its object or, applied to the eye, so long as the object 

 is more than twice the distance of the outer focal point from the eye, the 

 image on the retina is smaller than the object. This tallies with our experi- 

 ence that we cannot focus sharply on the retina rays from objects lying nearer 

 the eye than twice its focal distance. 



1 One diopter (D) is the refracting power of a lens with a focal distance of 1 meter: 

 the refracting power is the reciprocal of the focal distance. 



