THE EYE. 303 



rays from it is more abundant. We now com- 

 prehend how finely it is provided that the crys- 

 talhne lens should be surrounded with the liquor 

 Morgagni, a fluid which is but in a slight degree 

 more dense than the aqueous humour. The ex- 

 terior surface of the lens itself is only a little more 

 dense than the surrounding fluid, and each suc- 

 cessive layer, from the surface to the centre, is of 

 gradually increasing density : so that if we were 

 to describe the course of the ray, it would not, 

 as we see in the ordinary diagrams, pass like a 



straight line of the pen, but in a curved line, 

 showing the gradual manner in which the ray is 

 refracted through successive transparent layers. 

 As it enters in the anterior half of its passage, it 

 encounters media of increasing density : but as it 

 passes out behind, it is transmitted through media 

 diminishing in density. The ray is nowhere op- 

 posed by that sudden increase of density which 

 gives a disposition to reflection ; and it passes 

 through the vitreous humour still refracted, the 

 density of that humour having a just correspond- 

 ence with the posterior surface of the lens. In 

 the atmosphere there is a similar arrangement for 

 receiving the light proceeding from the sun or 

 stars : for as the density of the air diminishes as 



