140 Dr. C. A. Sadler and Mr. P. Mesham on Rontgen 



the path of the tertiary radiation. When testing the 

 secondary radiation readings were taken on the primary and 

 secondary electroscopes. By a suitable interpolation the 

 corresponding readings on the secondary electroscope and 

 the tertiary electrometer could be obtained at any time.) 



Calculations based on careful drawings gave the order of 

 correction to be expected, but its experimental determination 

 was deemed more satisfactory. For this purpose a plate of 

 pure copper was substituted for the carbon, and the absorp- 

 tion coefficient in aluminium of the oblique tertiary homo- 

 geneous radiation from it (excited by a more penetrating 

 secondary beam) was found and compared with a nearly 

 parallel homogeneous beam from copper when used as a 

 secondary. The correction obtained in this case was in very 

 close agreement with that obtained when other oblique 

 homogeneous tertiaries of a more penetrating type were 

 compared with the corresponding secondary parallel beams. 



It was found when absorption experiments were carried 

 out on the lines indicated above, that when all such cor- 

 rections had been applied there was strong evidence that the 

 radiation excited in carbon was heterogeneous, and distinctly 

 less pe net ratuiij than the primary exciting beam. 



The results are given in the following table: — 



Table I. 



Element giving 

 sec. radiation. 



Ve 



Cu 



Zn 



As 



Previous absorption i 

 of ter. radiation. 



nil 



50 p. cent. 

 80 p. cent. 



nil 

 50 p. cent. 



nil 

 24 p. cent. 

 49 p. cent. 



Percentage 

 absorption 

 of tertiary. 



1 

 Percentage 

 absorption 

 of secoudary. 



1 



26 7 



23-5 



32-4 

 292 



25-2 



253 

 209 



29 8 

 25 7 



21-8 

 18-9 

 164 



11-9 



Different thicknesses of aluminium were used as absorbers 

 for the different beams, in order to keep down the absorption 



