COHESION 33 



and other stones, would affect the light; and, if Dr. Tyn- 

 dall will be good enough to let us use his light again, we will 

 first of all show you how it may be bent by a piece of glass 

 (Fie. 19). [The electric lamp was again lit, and the beam 



FIG. 19 



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 show 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 ii 

 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 spec- 

 trum 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 dif- 

 ferent colors. 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 de- 

 grees 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 



HC TOL. XXX B 



