304 NATURAL THEOLOGY. ^ 



the height above the earth increases, the surface 

 of our atmosphere, from its rarity, must almost 

 resemble free space ; consequently the light falling 

 into it will penetrate more abundantly than if the 

 air were compressed as it is near the earth, and 

 were of uniform density. We thus see the obvious 

 superiority in the structure of the eye to any 

 thing that can be composed of glass, which is of 

 uniform density throughout, and must therefore 

 present a succession of surfaces where rare and 

 dense media are abruptly opposed to the rays 

 transmitted. 



We may observe another happy result from the 

 peculiar structure of the lens. A magnifying glass 

 is never true : an aberration of the rays takes place 

 in the pencil of light, as the rays are drawn to a 

 focus. The rays which penetrate near the centre 

 are projected so as to be drawn to their focus 

 beyond those rays which pierce through nearer 

 the edge. The rays penetrating the centre of 

 this double convex glass will project the image 



to A, whilst those penetrating nearer the circum- 

 ference, and consequently falling more obliquely, 

 will form a focus nearer the lens at b. But in 

 the crystalline humour of the eye, which corre- 

 sponds with the optician's lens, the exterior layer 

 having less density, and therefore a diminished 



