302 NATURAL THEOLOGY. 



water. The reason of this is, that the rays of 

 light are reflected when entering from a rare 

 medium into a denser, more abundantly in pro- 

 portion to the difference of the density. When 

 the ray of light has penetrated the water, it also 

 penetrates the glass, because there is not that 

 difference of density between the water and the 

 glass which there is between the atmosphere and 

 the glass. From this we may estimate the impor- 

 tance of the surface of the cornea being moistened 

 by the tears ; for however thinly the water may 

 be spread over the surface of the eye, it is suflfi- 

 cient to make those rays that would otherwise be 

 reflected penetrate the cornea. 



The whole humours of the eye are constituted 

 with a regard to this law. There is nowhere an 

 abrupt transition from a rare to a dense humour. 

 The ray is transmitted from the cornea into the 

 aqueous humour, and through that humour into 

 the lens or crystalline humour. Were this latter 

 humour uniform and of the density of its central 

 part throughout, the ray would be in part reflected 

 back from its surface. But it is not uniform, like 

 a mass of glass : it consists of concentric layers 

 increasing in density from the surface to the 

 centre. If we first look at the entire lens, and 

 then take off* its concentric layers, we shall see 

 the surface of the internal nucleus more distinctly 

 than the exterior and natural surface. The reason 

 is obvious : the nucleus is so much more dense 

 than the atmosphere, that the reflection of tiie 



