NATURAL THEOLOGY. 51 



it will be reflected, or thrown back in this manner; 

 — or, more generally, the principle is this : when 

 light passes from air into glass, or from water into 

 gl ass — or from any such substance into another of a 

 different kind, though most of it will be transmitted, a 

 portion will be stopped at the surface, and be thrown 

 back or reflected. The glistening upon the surface 

 of a bright window is this effect. The light, in pass- 

 ing from the air into the window, undergoes a reflec- 

 tion, in some measure, at the surface of the glass. It 

 is because the air and the glass are not the same sub- 

 stance. There is the same glistening upon the sur- 

 face of the ocean ; but there is none within the ocean 

 itself; because the water continues the same. How 

 it should fail to be quite considerable in the eye, 

 when there are no less than three different kinds of 

 substances through which the light must pass, has 

 been a question. It is stated that there is a most 

 curious and proyident contrivance which meets this 

 difficulty. Notwithstanding the three substances in 

 the eye differ, generally speaking, like air, water and 

 glass, though the difference is less ; yet, where they 

 touch, and where the light passes from one to the 

 other, and where it must be reflected, if reflected at 

 all, they are nearly of the same substance. The 

 middle one, which is the chrystalline, and which is 

 tmite solid toward the centre, diminishes its consis- 

 tency on its two opposite sides, to approach that of 

 the vitreous behind, and that of the aqueous before. 

 Thus, there is less chance for the light to be thrown 

 back or reflected from the glasses ; the principle of 

 the reflection requiring that the light, in passing from 



