THEORY OF LIGHT. 269 



we put aside the consideration of the living properties of the organ, as the 

 extraordinary variety and degrees of sensibility in the nerve of vision, 

 and confine ourselves to points easily comprehended, as, for example, 

 the mechanism of the eye, and the laws of opticts as applicable to the 

 humours, we shall find enough to admire. When we look upon the 

 optician's lens, however perfect its polish may be, we can see its 

 convex surface ; that is to say, the rays of light which strike upon that 

 surface do not all penetrate it, but are in part reflected to our eye, which 

 is the occasion of our seeing it. We do not see the surface of the corner 

 of the human eye. Here, then, is an obvious superiority, since it implies 

 that all the rays of light which strike the corner enter it and are refracted, 

 and none are returned to our eye. If we take the optician's lens between 

 our fingers and hold it under water, we can no longer see it, however 

 transparent the 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 proportion 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 

 importance 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 

 sufficient 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 the rays from it is 

 more abundant. We now comprehend how finely it is provided that the 

 crystalline lens should be surrounded with the liquor Morgagni^ a fluid 

 which is but in a slight degree more dense than the aqueous humour. 

 The exterior surface of the lens itself is only a little more dense than 

 the surrounding fluid, and each successive layer, from the surface to the 

 centre, is of gradually increasing density : so that if we were to describe 



