774 



Mr. H. Pealing on Distribution and Quality of 



of iron, so this explanation of the rise of the value of I150/I90 

 with hardness of the secondary is the more likely one. 



When the bulb is very soft indeed the proportion of iron 

 radiation present will be considerably increased, as for hard 

 beams the scattered radiation will not be absorbed by the 

 carbon to nearly the same extent as the iron radiation. Thus 

 we explain the fact that the ratio is low for very soft beams. 

 Table I. is typical of the results obtained when rays of 

 moderate penetrating power were used. 



Table T. 



Absorption of the secondary 



beam in direction 150° to 



primary beam by aluminium 



0'15 mm. thick. 



Value of 



Il50 



1~90 



for whole 



beam. 



Value of 



IJ50 



I90 



for beatn subsequent to 



absorption in column 1. 



307 per cent. 



29-0 



26-3 



1-62 

 1-60 

 1-54 



1-67 

 1-70 

 1-60 



This also shows clearly that the ratio drops as the rays 

 become harder, thus confirming the result predicted by theory 

 that as the beam became harder the ratio would get less owing 

 to the presence of hard fluorescent carbon radiation. When 

 the primary beam was cut down by aluminium before falling 

 on the radiator, somewhat different results were obtained. 

 These are shown in Table II. 



Table II. 



Thickness of aluminium 

 absorbing primary rays. 



Values of 

 Il50 



190" 



Thickness of aluminium 

 absorbing secondary rays. 



none. 

 0-95 mm. 

 3-2 mm. 



none. 



163 

 1-64 



1-68 

 1-41 



none, 

 none, 

 none. 

 3-2 mm. 



The figures in column 2, Table II. give the values of ^ 



J- 90 



observed when the primary has been deprived o£ its softer 

 constituents by the thickness of aluminium given in column 1. 



