92 SPECIAL SENSES. 



Calculating the curvatures of the refracting surfaces in 

 the eye and the indices of refraction of its transparent media, 

 it has been pretty clearly shown, by mathematical formulae, 

 that the eye, viewed simply as an optical instrument, and not 

 practically, as the organ of vision, presents a certain degree 

 of spherical and chromatic aberration ; 1 but with these for- 

 mulas we have little to do in our purely physiological con- 

 sideration of vision. As a matter of interest, however, we 

 give a series of calculations, according to the best authorities 

 upon the subject of physiological optics. 2 



In most calculations of the size of images, the positions 

 c.f conjugate foci, etc., in normal and abnormal eyes, a sche- 

 matic eye reduced by Donders, 3 after the example of Listing, 4 

 is regarded as sufficiently exact for all practical purposes. 

 This simple scheme represents the eye as reduced to a single 

 refracting surface, the cornea, and a single liquid assumed to 

 have an index of refraction equal to that of pure water. The 

 distance between what are called the two nodal points and be- 

 tween the two principal points of the dioptric system of the 

 eye is so small, amounting to hardly j-^g- of an inch, that it 

 can be neglected. In this simple eye, we assume a radius of 



1 For an exceedingly interesting account of the eye considered as an optical 

 instrument, see HELMHOLTZ, Uceil considers comme instrument cToptique. Revue 

 des cours scientifiques, Paris, 1868-1869, tome vi., p. 211, et seq. 



2 HELMHOLTZ, Optique physiologique, Paris, 1867, p. 90, after LISTING. 

 Index of refraction of the atmosphere, 1 



Index of refraction of the aqueous humor, -^f- 



Index of refraction of the crystalline, jf 



Index of refraction of the vitreous humor, -^- 



Radius of curvature of the cornea, O312 of an inch. 



Radius of curvature of the anterior surface of the crystalline, O390 " " 

 Radius of curvature of the posterior surface of the crystalline, 0'234 " " 

 Distance between the anterior surface of the cornea and the 



anterior surface of the crystalline, 0*156 " " 



Thickness of the crystalline, 0-156 " " 



3 DONDERS, On the Anomalies of Accommodation and Refraction of the Eye, 

 The New Sydenham Society, London, 1864, p. 176. 



4 LISTING, Dioptrik des Auges, in WAGNER, Handworterbuch der Physiologic, 

 Braunschweig, 1853, Bd. iv., S. 493, et seq. 



