562 



THE NERVOUS SYSTEM. 



The data here given are known as optical constants, and the 

 figures may be regarded as averages, individual eyes differing, as 

 would naturally be expected. 



Reduced Eye (Fig. 338). For purposes of calculation, an 

 imaginary eye, the reduced or schematic eye, has been proposed, 



FIG. 338. The reduced eye: S, the single spherical refracting surface, 1.8 mm. 

 behind the anterior surface of the cornea; N, the nodal point, 5 mm. behind S ; F, 

 the principal focus (on the retina), 20 mm. behind S. The cornea and lens are put 

 in in dotted lines in the position which they occupy in the normal eye (Stewart). 



whose refractive medium is a single one, representing, approxi- 

 mately enough for practical purposes, the natural eye. This eye 

 has the following characteristics (Listing) : 



From anterior surface of cornea to the princi] 

 From nodal point to the posterior surface of lens . . 

 Posterior principal focus behind cornea .... 

 Anterior principal focus in front of cornea . . 

 Kadius of curvature of single refracting surface 

 Index of refraction of single refractive medium 



2.3448 mm. 



0.4764 " 



22.8237 " 



12.8326 " 



5.1248 " 



1.33 " 



The optical and visual axes may be regarded as identical, and 

 as represented by a line which passes through the centers of cur- 

 vature of the cornea and lens directly backward until it terminates 



FIG. 339. Diagram of the formation of a retinal image (after Foster). 



in the fovea centralis. Rays of light falling upon the cornea are 

 refracted and made convergent, and this effect is increased by the 



