213 POPULAR LECTURES AND ADDRESSES. 



held the large slab of uranium glass in the ultra- 

 violet light of the prismatic spectrum. As long 

 as I held the uranium glass there you saw it 

 glowing ; the moment I took it out of the invisible 

 light it ceased to glow. The " moment " of which 

 we were then cognisant may have been the tenth 

 of a second. If the uranium glass had continued 

 to glow sensibly for the twentieth or the fiftieth of 

 a second, it would have seemed to our slow-going 

 sense of vision to cease the moment it was taken 

 out. Now I turn the wheel at such a rate that 

 the hole next you is open about a fiftieth of a 

 second after the uranium glass was bathed in light ; 

 still you see nothing. I turn it faster and faster 

 and it now begins to glow, when the hole next 

 you is open about the two-hundredth of a second 

 after the immediately preceding admission of light 

 by the other hole. I turn it faster and faster, and 

 it glows more and more brightly, till now it is 

 glowing like a red coal ; further augmentation of 

 the speed shows, as you see, but little difference in 

 the glow. 



Thus it seems that fluorescence is essentially 



