I have shewn you this experiment first, so that you might understand how glass attracts light, and might then see how other substances, like rock-salt and calcareous spar, mica, and other stones, would affect the light; and, if Dr. Tyndall will be good enough to let us use his light again, we will first of all shew you how it may be bent by a piece of glass (fig. 19). [The electric lamp was again lit, and the beam of parallel rays of light which it emitted was bent about and decomposed by means of the prism.] Now, here you see, if I send the light through this piece of plain glass, A, it goes straight through, without being bent, unless the glass be held obliquely, and then the phenomenon becomes more complicated; but if I take this piece of glass, B [a prism], you see it will shew a very different effect. It no longer goes to that wall, but it is bent to this screen, C; and how much more beautiful it is now [throwing the prismatic spectrum on the screen]. This ray of light is bent out of its course by the attraction of the glass upon it. And you see I can turn and twist the rays to and fro, in different parts of the room, just as I please. Now it goes there, now here. [The Lecturer projected the prismatic spectrum about the theatre.] Here I have the rays once more bent on to the screen, and you see how wonderfully and beautifully that piece of glass not only bends the light by virtue of its attraction, but actually splits it up into different colours. Now, I want you to understand that this piece of glass [the prism] being perfectly uniform in its internal structure, tells us about the action of these other bodies which 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 shewed 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 shew 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 after leaf, and see how that enables the light to pass through to the screen, and how, as Dr. Tyndall turns it round