McCietianp anp Hacxerr—The Absorption of B Radium Rays by Matter. 45 
Now, denoting by (7), the secondary radiation from the plate of thickness d on 
the same side as the incident radiation &,, we evidently have 
2rd __ 1 
r\=B+ B=bR, — = 
("0 > (A £ ale? + =a)? 
and denoting by (7,), the similar radiation when the plate is thick enough 
to absorb all radiation, we have 
1 
(@)), SF ilioy «a 8 
2rd d g 
P.)) A+a 
Co). = - a) 
(7o)a QoNe 1 
or 
GG, cs e 2A = ] 
(7), Ty (7o)a e 2A 
A+a 
or 
8 fe BY We on 
em PR). = (Pyle 
Viz +(1-5) ; ee 
1 (7) )a tes 
C (A). a (7)a’ 
where C’ depends only on «x, and, as stated earlier in the paper where the same 
quantity occurred, is calculated from the values of x for different substances 
given in the previous paper above referred to. 
We have, therefore, the coefficient ) by measuring the secondary radiation 
emitted from the face on which the primary radiation falls for a plate of small 
thickness ¢, and for a plate thick enough to stop all the radiation. 
It may be mentioned at once that the values of X found by these two very 
different methods agree fairly well, which is of importance, not merely as regards 
the coefficient itself, but as affording a severe test of the general accuracy of 
the theoretical treatment of secondary radiation given in this and in previous 
papers. Practically the same theoretical work was depended upon in calculating 
the relative energy of the secondary rays and in making several other deductions, 
and it is of some importance to confirm the accuracy of the work in the way 
described above. 
TRANS. ROY. DUB. SOC., N.S., VOL. IX., PART IV. I 
