THE SENSES 783 



of aqueous humour and corneal substance are nearly equal. 

 At the anterior and posterior surface of the lens the ray is 

 again refracted, since the refractive index of the aqueous and 

 vitreous humour is less than that of the lens. The following 

 tables show the radii of curvature of the refracting surfaces 

 and the refractive indices of the dioptric media, as well as 

 some other data which are of use in studying the problems 

 of refraction in the eye : 



In accommodation for 

 Far Vision. Near Vision. 



(Cornea - -7*8 mm. 7-8 mm. 

 Radius of curvature OH Anterior surface of lens io'o 6-0 

 (Posterior surface of lens 6'o 5-5 

 Anterior surface of cornea and an- 

 terior surface of lens - - 3-6 ., 3-2 



Distance 

 between 



Anterior surface of cornea and pos- 



terior surface of lens - - 7-6 7-6 



Anterior and posterior surface of lens 4-0 ,, 4*4 ,, 



Posterior surface of lens and retina - 14*6 ,, 14*6 



Antero-posterior diameter of eye along the axis 22-2 ,, 22-2 



Eefractive Indices 



Air - -i "ooo 



Cornea - - 1-377 



Aqueous humour ...... 1*3365 



Vitreous humour - ... 1-3365 



Lens (mean for all its layers) - - 1-437 



Water - - 1-335 



It will be seen that the refractive indices of the cornea and 

 the aqueous and vitreous humours are all nearly the same as 

 that of water. That of the lens differs for its various layers, 

 the central core having a higher refractive index than the 

 more superficial portions ; but a mean may be struck, and, 

 although such calculations are open to error, it has been 

 computed that the lens acts as a homogeneous lens of the 

 same curvatures, and with a refractive index of 1*437, 

 would do. 



The optical problems connected with the formation of the 

 retinal image are complicated by the existence in the eye of 

 several media, with different refractive indices, bounded by 

 surfaces of different and, in certain cases, of variable curva- 

 ture. For many purposes, however, the matter can be 

 greatly simplified, and a close enough approximation yet 



