Page:Various Forces of Matter.djvu/66

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