24 M‘Crevttanp—The Energy of Secondary Radiation. 
substance interposed, and the intensity again measured after passing through 
the plate. A coefficient of absorption is then calculated by assuming 
ey ay 
lik S 1G 
where f&, is the intensity of radiation after traversing the plate of thickness d, 
FR, is the incident radiation, and 2’ is the coefficient of absorption. This method 
does not give the coefficient of absorption accurately ; too great a value will be 
found when the thickness of the absorbing layer is very small, and the error 
will be greater in the case of substances which give strong secondary radiation. 
The coefficient determined in this way should diminish as the thickness of the 
absorbing layer increases, even in the case of a perfectly homogeneous radiation. 
The coefficient of absorption of 6 rays determined in this way has been found 
to decrease rapidly as the thickness traversed increases; and the result has 
been: taken to show the marked heterogeneity of the rays as regards 
penetrating power. Part of the observed decrease of the coefficient, but not 
all of it, is, however, due to the faultiness of the method, so that the rays are 
not so heterogeneous as such experiments might indicate. We see this as 
follows :— 
Using the notation of the previous section, the intensity of the downward 
flow of energy at depth zw of a plate which is not thick enough to prevent all 
transmission of radiation is 
Rez Ae tVe-? 4 Ale Var — 
We shall write X for 
Je — B ; 
and our object is to show how 2X differs from )’, determined as described above, 
and how the difference depends on the thickness d of the plate used. 
When G32 0, 
we have li, = Al db Als 
lig lige? do ANE? = eo), 
Using the equation (1) above, viz. : 
ian — ak + br, 
dx 
