Fig. 15., Fig. 16. and Fig. 17.

Now I want you to understand a little more how this is—and for this purpose I am going to use the electric light again. You see, we64 cannot look into the middle of a body like this piece of glass. We perceive the outside form, and the inside form, and we look through it; but we cannot well find out how these forms become so: and I want you, therefore, to take a lesson in the way in which we use a ray of light for the purpose of seeing what is in the interior of bodies. Light is a thing which is, so to say, attracted by every substance that gravitates (and we do not know anything that does not). All matter affects light more or less by what we may consider as a kind of attraction, and I have arranged (fig. 18) a very simple experiment upon the floor of the room for the purpose of illustrating this. I have put into that basin a few things which those who are in the body of the theatre will not be able to see, and I am going to make use of this power, which matter possesses, of attracting a ray of light. If Mr. Anderson pours some water, gently and steadily, into the basin, the water will attract the rays of light downwards, and the piece of silver and the sealing-wax will appear to rise up into the sight of those who were before not high enough to see over65 the side of the basin to its bottom. [Mr. Anderson here poured water into the basin, and upon the Lecturer asking whether any body could see the silver and sea