

THE SIZE OF A TOMS. 2 1 1 



escence ? has been answered affirmatively by this 

 beautiful and simple little machine before you 

 which he invented for the purpose. The experiment 

 giving the answer is most interesting, and I am sure 

 you will see it with pleasure. The apparatus con- 

 sists of a flat circular box, with two holes facing 

 one another in the flat sides near the circumfer- 

 ence ; inside are two disks, carried by a rapidly 

 revolving shaft, by which the holes are alternately 

 shut and opened ; one opened when the other is 

 closed, and vice versa. A little piece of uranium 

 glass is fixed inside the box between the two holes, 

 and a beam of light from the electric lamp falls 

 upon one of the holes. You look at the other. 



Now when I turn the shaft slowly you see 

 nothing. At this instant the light falls on the 

 uranium glass through the open hole far from you, 

 but you see nothing, because the hole next you 

 is shut. Now the hole next you is open, but you 

 see nothing, because the hole next the light is shut, 

 and the uranium glass shows no perceptible after- 

 glow as arising from its previous illumination. 

 This agrees exactly with what you saw when I 



P 2 



