34 FARADAY 



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 polarized 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 polarized 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 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 in his hand, you have those different 

 colors, pink, and purple, and green, coming and going most 

 beautifully; not that the mica is more transparent than the 

 glass, but because of the different manner in which its 

 particles are arranged by the force of cohesion. 



Now we will see how calcareous spar acts upon this light 

 that stone which split up into rhombs, and of which you 

 are each of you going to take a little piece home. [The 

 mica was removed, and a piece of calc-spar introduced at 

 A.] See how that turns the light round and round, and 



m 



FIG. 20 



produces these rings and that black cross (Fie. 20). Look 

 at those colors : are they not most beautiful for you and for 

 me? (for I enjoy these things as much as you do). In what 



