262 HUMAN PHYSIOLOGY 



The nodal, chief, and focal points of the eye are collectively 

 called the cardinal points (cf. infra). 



The second focal point of the eye lies 22.23 mm beind the 

 vertex of the cornea, the first focal point 12.92 mm (/ 2 and 

 /i, Fig. 28). The second focal plane nearly coincides with 



FIG. 28. CARDINAL POINTS OF THE EYE (see text). 

 GG' is the visual axis. 



the retina. Images of infinitely removed objects having 

 parallel rays fall on the retina. Hence the normal retina can 

 clearly see objects at an infinite distance. 



II. Accommodation. The image of a near luminous point 

 falls behind the retina of the resting eye and no image is 

 formed on the retina. In its place a luminous circle 

 appears, the circle of diffusion, because the rays have not 

 yet united. As the circles of diffusion of a near luminous 

 point overlap each other the image of a near object on the 

 retina is not sharply defined, hence the resting eye cannot 

 clearly see near objects. 



In order to form upon the retina a sharply defined image 

 of a near object, the refractive power of the eye is increased 

 by increasing the curvature of the surfaces of the lens. This 

 process is called accommodation. 



