532 PHYSIOLOGY 



surfaces at a certain distance apart. The rays falling upon the first surface are not 

 refracted from it, but are regarded as projected forwards parallel with themselves to 

 the second surface. Refraction is then considered to take place at the second surface 

 just as if that were the only surface present (represented by the dotted line II in 

 Fig. 262). 



FIG. 262. The position of the cardinal points in the schematic eye. (HELMHOLTZ.) 

 h,, h,,, principal points ; k,, k,,, nodal points ; F,,, posterior focus. 



All such systems possess three pairs of cardinal points, viz. two principal 

 foci, two principal points, and two nodal points. These points may be 

 denned by the following principles : 



(a) Rays which pass through the principal focus are, after refraction, 

 parallel to the optic axis. 



(b) Rays which pass through the first principal point, after refraction, 

 pass through the second. 



(c) Rays which pass through the first nodal point, after refraction pass 

 through the second, and the direction of the refracted ray is parallel to the 

 direction of the incident ray. 



In Fig. 262 the situation of these cardinal points is shown in the human 

 eye. The rays of light pass through the following media, cornea, aqueous 

 humour, lens, and vitreous humour, and may be refracted at the following 

 surfaces ; anterior and posterior surfaces of the cornea, anterior and posterior 

 surfaces of the lens. Since, however, the refractive indices of cornea, 

 aqueous humour, and vitreous humour are practically identical, we can 

 reduce the refractive media to two, viz. cornea, aqueous, and vitreous 

 in the one case, and lens in the other ; and the refractive surfaces to three, 

 viz. anterior surface of cornea, anterior surface of lens, and posterior surface 

 of lens. 



In order to determine the path of the rays in the eye we have to deter- 

 mine the following data, viz. : 



(1) The radius of curvature of the anterior surface of the cornea. 



(2) The distance of the anterior surface of the cornea from the anterior 

 surface of the lens. 



