262 



between the source and sulphide screen. In the photographs it is seen 

 that there is a gradual decay in the luminous intensity of the screen, 

 and if there is any radiation coming from the burner, in no case is it 

 sufficently intense to overcome the decay or even make the rate notice- 

 ably different. 



Photograph IV was taken by exposing one-half of the luminous 

 screen to the radiations while at the same time the other half, which 

 was screened from them by lead, was exposed. The arrangement is 

 similar to that shown in Fig. III. 





S is a large lead plate 1 nmi. thick \<\X\\ a circular opening in the 

 center, on the back of which is fastened the sulphide screen. In the 

 line A D across the opening is a lead strip projecting 2 or 3 mm. toward. 

 A B C D is a small lead plate on the back side of the larger one. covering 

 one-half of the opening. With this arrangement sixteen exposures were 

 taken on one plate and a direct comparison can be made. In the sixteenth 



W. 



FIG. fl* . 



there is not much difference between the half marked N and the half not 

 marked at all. 



