44. McCrenianp anp Hacxurt—The Absorption of B Radium Rays by Matter. 
Seconp MeEruop. 
A second method of determining the coefficient \ was employed in a few 
experiments, and it is convenient to describe this method here before entering 
on an account of experimental results. 
It is possible to determine \ by observations of the secondary radiations 
from the plate on the side on which the primary rays are incident. The 
coefficient of absorption is thus determined without any measurements being 
made of the radiations that have actually passed through the plate. 
Taking the equations above: 
Ra Agr aes? 4 Aleve -#? 
y) 
pOR e — «Va? = & 1 aVa> — b? 
(Pp = 1B + Bre ; 
or 
ips AGr™ & Aree 
y = Be~*” iL, Biome 
and 
dr 
— = ar — OR. 
dx 
Using first a plate of thickness d, where dis small so that the radiation 
is not all absorbed, we have 
ps0 wren e= de 
Be~*™ AL Bem = (). (4) 
And from the equation 
when «= 0, we have 
(A+ a)B-(X-a)B’=6R,, (5) 
where &, is the incident radiation. 
From (4) and (5) we get 
gu 21 
153 So JB = bi. ——— ——— 
ms (A + a)e”” + (A - a) 
