REFRACTION IN THE EYE 439 



known that the error introduced by assuming 

 the lens to be homogeneous is unimportant. 

 Thus the simplified dioptric system of the eye 

 consists of three refracting surfaces : the anterior 

 surface of the cornea, 1 the anterior surface of the 

 lens, and the anterior surface of the vitreous 

 humor. The index of refraction of the aqueous 

 and vitreous humors is practically the same. 



The several refracting surfaces of this optical 

 system are approximately " centred," i. e. placed 

 with their centres of curvature upon a right line, 

 the optical axis. The diaphragm (iris) is of such 

 a size and position that the rays entering the eye 

 intersect the axis at small angles ; the aperture 

 of the system is therefore small. Under such 

 conditions it is possible to find upon the princi- 

 pal axis of the system certain cardinal points, 

 discovered by' Gauss, by the aid of which the 

 situation and size of the visual images may be 

 determined. The cardinal points are (1) the an- 

 terior principal focus, (2) the anterior principal 

 point, (3) the posterior principal point, (4) the 

 anterior nodal point, (5) the posterior nodal point, 

 (6) the posterior principal focus. These points 

 are reciprocal. 



As the dioptric system of the eye consists of a 



1 For convenience, the anterior surface of the cornea will be 

 held to include the layer of tears. 



