THE SENSES. 711 



It is quite obvious that the eye, even in the simplified form above 

 indicated, is a much more complicated optical apparatus than the one 

 described in the figure. It is, however, possible to reduce the refractive 



f 



Fig. 420. Diagram of a simple optical system (after M. Foster). The curved surface, 6, d, 

 is supposed to separate a less refractive medium toward the left from a more refractive medium 

 toward the right. 



surfaces and media to a simpler form when the refractive indices of the 

 different media and the curvature of each surface are known. All of 

 these data have been very carefully collected. They are as follows : 



Index of refraction of aqueous and vitreous = 1.3365 



lens . = 1.4371 



Radius of curvature of cornea . . . = 7. 829 mm. 



anterior surf ace of lens = 10 



posterior = 6 

 Distance from anterior surface of cornea and 



anterior surface of lens . . . . = 3.6 " 

 Distance from posterior surface of cornea and 



posterior surface of lens . . . = 7.2 " 



With these data, it has been found comparatively easy to reduce by 

 calculation the different surfaces of different curvatures, 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 dimensions : 



From anterior surface of cornea to the princi- 

 pal 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 " 



In this reduced or simplified eye the principal posterior focus, about 

 23 mm. behind the spherical surface, would correspond to the position 

 of the retina behind anterior surface of cornea. The refracting surface 

 would be situated about midway between the posterior surface of the 

 cornea and the anterior surface of the lens. 



The optical axis of the eye is a line drawn through the centres of 



