54 COHESION. 



are not uniform which do not merely cohere, 

 but also have within them, in different parts, 

 different degrees of cohesion, and thus attract 

 and bend the light with varying powers. We 

 will now let the light pass through one or two 

 of these things which I just now showed you, 

 broke so curiously ; and, first of all, I will take 

 a piece of mica. Here, you see, is our ray of 

 light we have first to make it what we call 

 polarised, but about that you need not trouble 

 yourselves, it is only to make our illustration 

 more clear. Here, then, we have our polarised 

 ray of light, and I can so adjust it as to make 

 the screen, upon which it is shining, either 

 light or dark, although I have nothing in the 

 course of this ray of light but what is perfectly 

 transparent [turning the analyser round], I 

 will now make it so that it is quite dark, and 

 we will, in the first instance, put a piece of 

 common glass into the polarised ray so as to 

 show you that it does not enable the light to 

 get through. You see the screen remains 

 dark. The glass then, internally, has no effect 

 upon the light. [The glass was removed, and 

 a piece of mica introduced.] Now, there is the 

 mica which we split up so curiously, into leaf 



