roiS 



THE SENSES 



Refractive Indices 



Air 



Cornea - - - - 

 Aqueous humour 

 Vitreous humour - 

 Lens (total refractive index) - 

 Water -. 



I-OOO 



1-377 



1-3365 



1-3365 



1-437 



1-335 



It will be seen that the refractive indices of the aqueous and vitreous 

 humours are nearly the same as that of water. That of the lens 

 differs for its various layers, the central core having a higher re- 

 fractive index (1-411) than the more superficial portions (1-388). 

 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. This is called the 

 total refractive index of the lens.. The 

 apparent paradox that it is greater than the 

 refractive index even of the core is explained 

 by the consideration that the core taken by 

 itself has a greater curvature than the entire 

 lens, and therefore causes a greater amount of 

 refraction in proportion to its refractive index. 

 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 curvature. For many purposes, how- 

 ever, the matter can be greatly simplified, and 

 a close enough approximation yet arrived at, 

 by considering a single homogeneous medium, 

 of definite refractive index, and bounded in 

 front by a spherical surface of definite curva- 

 ture, to replace the transparent solids and 

 liquids of the eye. The principal focus being supposed to lie 

 on the retina, the position of the nodal point i.e., the point 

 through which rays pass without refraction of such a ' reduced ' 

 or ' schematic ' or ' simplified ' eye, and other constants, are shown 

 in the following table. The single refracting surface would be 

 situated behind the cornea and in front of the lens, at a rather 

 smaller distance from the anterior surface of the latter than from 

 the anterior surface of the former. The nodal point would be less 

 than half a millimetre in front of the posterior surface of the lens 

 (Fig. 419). The refractive index of the single transparent medium 

 would be a little greater than that of water. 



Fig. 419. The Reduced 

 Eye. S, the single 

 spherical refracting 

 surface, 2-2 mm. be- 

 hind the anterior sur- 

 face 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. 



