756 



THE EYE AND VISION. 



[CH. LV. 



however, possible to reduce the refractive surfaces and media to 

 a simpler form when the refractive indices of the different media 



Iig. 576. 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. 



and the curvature of each surface are known, 

 follows : 



Index of refraction of cornea 



,. aqueous and vitreous 



lens 



Radius of curvature of cornea 



anterior surface of len 



,, posterior 



Distance from anterior surface of cornea 



to anterior surface of lens 

 Distance from posterior surface of cornea 



to posterior surface of lens 

 Distance from posterior surface of lens to 

 retina 



These data are as 



= * '37 



= i -34 to 1-36 



_ ( I "4 in outer to 1-45 



\ in inner part. 

 = 7-8 mm. 



= 10 

 = 6 



= 3'6 

 = 7'2 



- 15-0 



With these data, it has been found comparatively easy to 

 reduce by calculation the different surfaces of different curva- 

 ture, 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 prin- 

 ciple, stiggested by Listing as the reduced eye, has the following 

 dimensions : 



From anterior surface of cornea to the 



principal point .... = 2*3448 mm. 



From the nodal point to the posterior sur- 

 face of lens . . . = -4764 mm. 



Posterior chief focus lies behind cornea . = 2 2 '82 3 7 mm. 



Anterior chief focus in front of cornea = 12 '83 2 6 mm. 



Radius of curvature of ideal surface . = 5' 1248 mm. 



