GENERAL ADAPTATION 25 



index of refraction which seem calculated to project 

 a distinct image on the expansion of the optic 

 nerve. 



Light, and light alone, is capable of doing this. 

 It is that which not only causes the coloured spots, 

 the first sign of the organ of vision, but, as adapta- 

 tion, it causes the image to be clearer each time, and 

 it is this which makes the expansion of the optic 

 nerve and the coloured spots act simultaneously 

 in the completion of the work of adjustment or 

 correspondence. As these spots go on increasing 

 through the action of the light, so their thickness 

 is increased ; and having then to withdraw from the 

 optic expansion, the light, to continue its relation 

 with it, has to refract in passing through the 

 lenticular thickness. This refraction has to be 

 according to the laws of optics, and therefore the 

 lenticular thickness possesses a radius and an 

 index of refraction which is that of the lens, and it 

 always corresponds with the optic nerve, without 

 which requisite, vision would be impossible. 



The organ of sight, like every organisation, obeys 

 the general laws of mechanics. Attention must be 

 drawn to the fact that in this sense its architecture 

 is ruled by the laws of optics, and it is not that 

 the crystal accommodates itself to these mechanical 

 conditions of light, but, on the contrary, these 

 mechanical conditions mould the organic substance, 



