Chap. XXIX.] 



OPTICAL CONSTANTS. 



377 



lens that brings the focal point forwards so as to fall 

 on the retina. The crystalline lens refracts the rajs 

 more than once, first by its anterior surface, when the 

 rays enter it from the aqueous humour, and last by its 

 posterior surface, when the rays issue from it to pass 

 into the vitreous body. But it has been shown 

 to be composed of various layers with different 

 densities and, consequently, different indices of 

 refraction, so that even while passing through the 

 substance of the lens rays of light will undergo a series 

 of successive refractions, all tending to converge the 

 rays to a focus. Thus rays of light in passing through 

 the eye encounter various media, with different 

 refractive indices, and the determination of the path 

 of the rays is to some extent complicated. We shall 

 therefore consider first the method of determining the 

 course of the rays in any system of refractive media, 

 and then apply the method to the particular case of 

 the human eye. 



In a system of several different refractive 

 media the path of a ray of light may be found by a 



Fig. 170. Construction of an Image by means of the Cardinal Points. 



geometrical construction. In Fig. 170 let ab CD and 

 El be spherical surfaces separating four different 

 refractive media, 1,2,3,4, and let the centres of curva- 

 ture of the media be in the same straight line, the line 

 passing through FF', which is called the principal 

 axis, the admission of six cardinal points or optical 



