LECT. XX. ACTION OF THE EYE ON LIGHT. 



365 



The numbers which represent the indices of refraction 

 for the media of the eye, are as follows: the index of re- 

 fraction of the aqueous humour differs but little from that of 

 water, which is 1-336 ; that of the aqueous humour is 1-337; 

 of the vitreous humour 1-3394; of the surface of the crys- 

 talline lens 1-3767; of the centre 1-399 ; and the mean, 

 1-3839. 



Eyes of other Animals. Some differences are observed 

 between the eyes of animals and those of man. In some 

 birds, the crystalline is almost spherical ; and, in all, the 

 transparent cornea is very convex. In fishes, on the con- 

 trary, the cornea is almost plane. The choroid,, also, pre- 

 sents very different colours in different animals. 



.fiction of the Eye on Light. Although the descrip- 

 tion of the eye, which I have here given you, is as sum- 

 mary as possible, yet, by its aid, I trust that you will easily 

 comprehend, in a general way, the progress of the rays of 

 light through the eye, by considering that this ougan is con- 



Fig. 32. 



Diagram explanatory of the Mechanism of Vision. 



stituted by a system of spherical lenses, which produce the 

 convergence of the luminous rays. 

 Most of the says of the luminous cxwies emitted by the 



