p. LECOMTE DU NOUY 



755 



in which I = emergent light, /o = incident light, K 

 n = the number of screens interposed, leads to this: 

 Log Ik = Log I — n Log K 



h, 



and 



and as ii = 10 

 whence 



Log In = Log Zo — » 

 In = 7o 10-« 



The number of screens interposed will itself give the order of mag- 

 nitude of the out-coming energy: 2 screens will mean that the energy 

 is decreased by 100; 4 screens, by 10,000, etc. 



It was found that especially prepared white paper fulfilled the 

 requirements better than any other screen. Sheets of the same paper 

 were chosen, (mean thickness 0.09 mm.), and placed exactly in front 

 of the thermopile, then the throw of the galvanometer was observed; 

 the paper was removed and another reading made at three different 

 wave lengths. A great number were tested, and as we were unable 

 to get ten sheets exactly similar, the thicker were placed on a plane 

 surface and evenly rubbed down with very thin sand, then glossed 

 again with a piece of round glass. They were frequently tested 

 during this process, and finally the following results were considered 



as satisfactory. (The figures express the ratio — ). 



TABLE TI. 



