730 Mrs. & Mr. Soddy and Mr. A. S. Russell on the 



21 

 worked out for cos = - (0 = 17° 45'). In the following 



It 7 I 



table the values of ~, and ~, the latter calculated from the 



simple exponential formula, are contrasted for various values 

 of XT :— 



At. 



It' 



It 



A 7 A. 





lo' 



lo" 





•00 



100000 



1 '00000 



1-02406 



•20 



•81480 



•81873 



102473 



•40 



•66382 



•67032 



1 02519 



•GO 



•54075 



•54881 



102432 



•80 



•44058 



•44933 



1-02382 



2-0 



•12896 



•13534 



102145 



40 



•016674 



•018316 





In the last column X'/X represents the relation of the 

 apparent to the true absorption coefficient, X' being the value 

 that would be obtained for the absorption coefficient if 

 calculated for the two values of XT indicated in the table 

 according to the simple exponential law instead of from 

 equation (5). It will be seen, therefore, that the general 

 effect of using a cone of rays of semi-angle 18° is that the 

 absorption coefficient experimentally obtained is from 2\ to 

 2 per cent, too high throughout the whole of the range. Thus 

 the obliquity of part of the beam is completely insufficient to 

 account for the initial irregularities of the absorption curves. 



Diagram B. 



Equation (4) assumes the plate to be of infinite area, and 

 it is scarcely possible working with the 7-rays of radium to 



