208 POPULAR LECTURES AND ADDRESSES. 



before it comes into the visible rays, are manifestly 

 invisible so far as the screen receiving the spectrum 

 is a test of visibility ; because the uranium glass, 

 and my hands holding it, throw no shadow on the 

 screen. Also you see the uranium glass which 

 I hold in my hand in the ultra-violet light, while 

 you do not see my hand. I now bring it nearer 

 the place where you see the air (or rather the 

 dust in it) illuminated by the violet light : still 

 no shadow on the screen, but the uranium glass 

 in my hand glowing more brilliantly with its 

 green light of very mixed constitution, consisting 

 of waves of longer periods than that of the ultra- 

 violet, which the incident light, of shorter period 

 than that of violet light, causes the particles of 

 the uranium glass to emit. This light is altogether 

 unpolarised. It was the absolute want of polari- 

 sation, and the fact of its periods being all less 

 than those of the exciting light, that led Stokes 

 to distinguish this illumination, which you see 

 in the uranium glass 1 from the mere molecular 



1 The same phenomenon is to be seen splendidly in sulphate of 

 quinine. An interesting experiment may be made by writing on 



