74. Prof. J. A. McClelland on the 
f Fig. 4 shows the result of experiments with a number of 
substances, the numbers along the horizontal axis expressing 
the thickness of the substance traversed in millims., and the 
numbers along the vertical axis denoting the corresponding 
intensity of the y rays in arbitrary units. 
Hig. 4. 
100 
80 
60 
30 
The form of the curves is not accurately given by the 
equation 
Teer, 
I being the intensity of the rays after passing through 
thickness v, I, the initial intensity, and A» the coefficient of 
absorption. To represent the curves by an equation of this 
type we must suppose ) to diminish with the thickness of 
substance traversed. These remarks apply chiefly to the curves 
for zinc, lead, mercury, and platinum. The absorption in 
the case of water, glass, and aluminium is so small, that 
sufficient accuracy to show any change in the coefficient of 
absorption along the curves could not be claimed. 
The coefficient of absorption has been calculated for each 
substance :—(i.) for the first 2°35 mms. of substance traversed ; 
(i.) for the thickness 2°5 to 5 mms.; (iil.) for the part 5 to 
10 mms.; (iv.) 10 to 15 mms. The results are shown in 
Table A. 
