288 



PHYSIOLOGY 



CHAP. 



the distance of these from each other and from the retinal surface, 

 also presents many difficulties in the living eye, since the measure- 

 ments on longitudinal sections through the hardened eyeball of 

 the cadaver do not correspond with the true proportions. Here 

 we can only give in the form of a table the extremes cited by the 

 most competent observers : 



Thickness of cornea in its central portion . . 0-45-1-37 mm. 



Depth of anterior chamber 2-90-4-09 



Thickness of lens . . . . . 3-03-4-43 



Distance of back of lens from retina . . . 15-00 mm. (average), 



According to the mathematical theory of Gauss (1841) any 



FIG. 128. Position of the six cardinal points in Helrnholtz' schematic eye. (Explanation in text.) 



kind of centred dioptric system, no matter how complicated, may 

 be replaced by a system of six cardinal points, as stated above. 

 Moser (1844) first applied this theory to the eye, to determine 

 the position of the two nodal points. Although the available 

 data for the optic constants of the eye were still very imperfect, 

 Listing (1847) not only completed the theory but ingeniously 

 constructed a " schematic eye," which differed but little from that 

 constructed by Helmholtz a few years later from the average of 

 measurements made upon the living eye. 



Helmholtz' schematic eye (Fig. 128) was constructed from the 

 following averages of the optic constants of the eye : 



Index of refraction of the air = 1-00 



Index of aqueous humour and vitreous body = 1-33 



