48 McCrenianp anp Hackert—TZhe Absorption of B Radium Rays by Matter. 
we calculate the coefficient \’ from the equation 
Ry = Roe Ae 
we get the values 58 and 53°5 respectively for the two plates. Calculating the 
coefficient 4 as explained above (equation 3), we get the values 48 and 47 
respectively. [It is important to notice how different the values of \’ would have 
been had we merely taken the readings in the position @ = 0, as is usually done. 
Instead of the ratios 140°5 : 89 and 140°5 : 60 for the intensity before and after 
passing through the plates, we would have had the ratios 1000 : 204 and 1000: 115. | 
The values for X obtained from plates of different thicknesses are therefore in 
closer agreement than the values of \’.. Had we used thinner plates, the difference 
between the values of \’ would have been much greater, and even the corrected 
values \ would have differed considerably. Our correction (equatiou 3) removes 
the influence of secondary radiation effects, or rather their relative effects when 
plates of different thicknesses are used, but does not, of course, take into account 
the want of homogeneity in the radiation itself. The correction from }’ to X is, 
however, of great importance, especially in the case of substances of high atomic 
weight for which the quantity « (involved in equation 3) is large. 
We shall now give the results of the second method described above in 
the case of zinc. When using this method, observations were not made for 
different values of #, and the total “reflected” radiation calculated as in the 
first method dealing with “transmitted” radiation. Measurements were made 
at one definite angle—usually 60°—for a thin plate, and for a plate thick enough 
to stop all the rays, and the coefficient ) was thus obtained as described above. 
It would, no doubt, have been more accurate to take the measurements for 
various values of #, as in the first method ; but the work is laborious, and the error 
introduced by the shorter method is probably not very great. 
The following numbers were obtained with zinc plates; the numbers are 
expressed in an arbitrary scale, but only ratios are used in the calculation :— 
Thickness of Zinc. Radiation, 
‘00786 cm. 145 
‘0159 em. 199 
A thick plate. 238 
The first and third numbers give for \ the value 54:6, and the second and 
third the value 54:2. The agreement between the values of ) obtained by the two 
methods is not very good in the case of zinc; it is much better in the case of some 
other substances, as the table below shows. However, considering the different 
character of the two methods, the agreement, even in the case of zinc, is quite 
satisfactory, and proves the accuracy of the theoretical treatment of the subject. 
