288 



PHYSIOLOGY 



CHAP. 



the distance of these ['mm each other and 1'mm tin- retinal surfa. e, 

 also presents many difficulties in the living ryr, since the meusure- 

 meuts on longitudinal sections through the hardened eyeball of 

 the cadaver do not cnnvspund with the true proportions. Here 

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

 most competent observers: 



Thickness of cornea in its central portion 

 Depth of anterior chamber . 



Thickness of lens .... 

 Distam < dl Kai-k of lens from retina 



0-4:. ]:*; jinn. 

 2-90 4-09 

 3-OH 4- 13 

 15-00 linn, (average). 



According to the mathematical theory of Gauss (1841) any 



Kic. 128. Position of the six canlinal points in Ili-linlmltx' schematic i-yc. (Explanation in Irxl.) 



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 Helrnholtz 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 



