48 
PHYSICS: DUANE AND MAZUMDER Proc. N. A. S. 
6. Evidently by means of the relation between the absorption and the 
wave-length it becomes possible in any practical case to determine the 
"effective" wave-length for a beam of X-rays by simply measuring the 
fraction of the radiation that passes through a plate of aluminium or a 
plate of copper. The details of such experiments will be given elsewhere. 
It is sufficient to note here that owing to the rapid change of absorption 
The Mass Coefficients of Ab5or)bt)on of 
X~ReiLj5. Aluminium and Coj^jDer 
0 .OOS .010 .0(5 
Th e Cube of the Wave Length, A . 
with wave-length, copper should be used for estimating the wave-length 
of short rays, whereas aluminium is better for longer ones. 
In certain practical cases (in radio-therapy, for instance) another 
method of estimating the "effective" wave-length appears to be preferable. 
This consists in determining by means of an ionization chamber, a photo- 
graphic plate or a fluorescent screen the thickness of aluminium that is 
