Vol.. 8, 1922 PHYSICS: DUANE AND MAZUMDER 47 
Aluminium plates of density 2.70 and plates of pure copper of density 
8.92 were employed. 
The following table contains the data obtained : 
ALUMINIUM 
COPPER 
.1653 
.218 
1.015 
.1477 
.194 
.745 
.1381 
.189 
.631 
.1290 
.182 
.538 
.1124 
.167 
.399 
.0951 
.161 
.299 
Our data agree very well with the values obtained by Richtmyer for 
wave-length .135. 
4. In the figure we have plotted the mass coefficients against the cubes 
of the wave-lengths. Points marked (+) represent our measurements. 
Those marked (o) represent Richtmyer's data. 
It appears that neither curve should be regarded as strictly a straight 
line. Both curves bend downward in the region of the shorter wave- 
lengths. Owing to this curvature it is impossible to estimate with cer- 
tainty the points at which the curves strike the axis of zero wave-length. 
Extrapolations of this kind may lead to quite erroneous conclusions. The 
curves, however, appear to be pointing toward the values .146 for alu- 
minium and .13 for copper at zero wave-length. 
According to the usual method of analyzing coefficients of absorption 
into fluorescent and scattering coefficients these may be regarded 
as mass scattering coefficients for aluminium and copper, respec- 
tively. It is quite possible, however, that, according to some of 
the recent conceptions of absorption, this division of the coefficients into 
fluorescent and scattering coefficients may be somewhat artificial. 
5. The following equations represent approximately the coefficients 
of absorption that we measured in the region of short wave-lengths : 
For aluminium - = 15.5 + .147 
For copper ^ = 193 + .13 
P 
It will be noticed that the mass scattering coefficient for aluminium does 
not differ appreciably from that obtained by Richtmyer, and that the 
mass scattering coefficient for copper agrees well with that given by Hull 
and Rice. The coefficients of the terms containing the cube of the wave- 
length, however, are each larger than the corresponding quantities deduced 
by Hull and Rice and Richtmyer from their experiments on longer waves. 
