THE SENSES 



Refractive Indices 



Air- 



Cornea - 



Aqueous humour 



Vitreous humour - 



Lens (total refractive index) - 



Water ----- 



903 



I'OOO 



1-377 

 1*3365 

 I '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 refractive index (1-411) than the more superficial por- 

 tions (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 exis- 

 tence 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 approxi- 

 mation yet arrived at, by considering a single homogeneous 

 medium, of definite refractive index, and bounded in front by a 

 spherical surface of definite curvature, 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. 385). The refractive index of the single trans- 

 parent medium would be a little greater than that of water. 



FIG. 385. THE REDUCED EYE. 



S, the single spherical refract- 

 ing surface, 2 '2 mm. behind the 

 anterior surface of the cornea ; 

 N, the nodal point, 5 mm. be- 

 hind 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. 



