442 
PHYSICS: C D. MILLER 
The electrical connections are shown in figure 2. The cathode current 
from the tube was balanced by the current from the dry cell, when the 
resistajice R was properly adjusted, by throwing the three point 
key to the right, and the galvanometer indicated when a balance was 
obtained. 
The absorbing material consisted of sheets of gelatin, celluloid, alum- 
inum foil and ordinary sheet aluminum. As the ratios between the 
absorption coefficients in these substances were found to be independent 
of the nature of the X rays, the observed absorption coefficients in 
gelatin and in celluloid have been reduced to equivalent absorption 
coefficients in aluminum, and the thickness of absorbing material 
through which the X rays past before the absorption coefficient was 
observed, has been similarly reduced to the equivalent thickness of 
aluminum. The ratios used for this reduction are 
(m/p) celluloid ^ ^ jfi/p) gelatin ^ ^ 
(/x/p) aluminum ' ' (m/p) aluminum 
fjL represents the absorption coefficient and p represents density, hence 
m/p is the mass-absorption coefficient. 
The results of the investigation are shown graphically in figure 3. 
The values of fi/p for the six higher potentials manifestly come to a 
steady value with increasing thickness of absorbing material. 
In figure 4 are plotted log V and log ju/p in which the values of /x/p 
are the Hmiting values, for the higher potentials, and the lowest values 
obtained, for the lower potentials. The points for the six higher poten- 
tials He very close to a straight line, the slope of which indicates 
^ F^'" = constant. 
P 
The other points do not represent the Hmiting values of ju/p, and hence 
would be expected to lie above this Hne. The mean value of the con- 
stant is 4.24 X 10^2; hence 
M _ 4.24 X 10^^ /.X 
p * 
The six values of ju/p as given by this formula agree to within less than 
one per cent with the observed values. 
If the 'end' radiation, which is homogeneous according to the 
absorption test, has a definite frequency given by the quantum theory 
relationship 
