CH. LVIII.] THE SCHEMATIC EYE 821 



that they are refracted in a line parallel with the axis d /"; this 

 point (fig. 520, F 2 ) is called the chief anterior focus. The optic axis 

 cuts the surface at what is called the principal point. 



FIG. 520. Diagram of a simple optical system (after M. Foster). The curved surface, b, d, is supposed 

 to separate a less refractive medium towards the left from a more refractive medium towards the 

 right. 



It is quite obvious that the eye is a much more complicated 

 optical apparatus than the one described in the figure. It is, how- 

 ever, possible to reduce the refractive surfaces and media to a simpler 

 form when the refractive indices of the different media and the 

 curvature of each surface are known. These data are as follows : 



Index of refraction of cornea . . . . = 1'37 



aqueous and vitreous . = 1 "34 to 1 '36 



, _ /I '4 in outer to 1 '45 



- \ in inner part. 



Radius of curvature of cornea . . . . = 7 '8 mm. 



anterior surface of lens = 10 



posterior . . . = 6 

 Distance from anterior surface of cornea to 



anterior surface of lens . . . . = 3'6 

 Distance from posterior surface of cornea to 



posterior surface of lens . . . . = 7*2 

 Distance from posterior surface of lens to 



retina = 15 '0 



With these data it has been found comparatively easy to reduce 

 by calculation the different surfaces of different curvature into one 

 mean curved surface of known curvature, and the differently refracting 

 media into one mean medium the refractive power of which is known. 



The simplest so-called schematic eye formed upon this principle, 

 suggested by Listing as the reduced eye, has the following dimen- 

 sions : 



From anterior surface of cornea to the principal point = 2'3448 mm. 

 From the nodal point to the posterior surface of lens *4764 



Posterior chief focus lies behind cornea . . . = 22'8237 ,, 



Anterior chief focus in front of cornea . . . = 12'8326 ,, 



Radius of curvature of ideal surface . . . = 5*1248 



The term index of refraction means the ratio of the sine of the 



