200 
PHYSICS: DUANE AND SHIMIZU 
The absorbing screen of lead was placed between the X-ray tube and the 
spectrometer. Professor Richards' laboratory kindly furnished us with the 
specimens of lead salts. Professor Richards has made accurate measurements 
of the atomic weights of lead isotopes, and the values he obtained for the two 
specimens we used were 207.20 for the ordinary lead and 206.08 for the radio- 
active lead. These differ from each other by more than J%. 
In our experiments the X-rays came from a molybdenum target tube of 
the Coolidge type, and a constant difference of potential amounting to about 
36,000 volts drove a current of 2 milliamperes through it. 
The curves in the figure represent the ionization currents as functions of 
the readings of one of the verniers attached to the crystal table. 
The three sharp drops in each curve correspond to the three critical ab- 
sorption wave-lengths belonging to the L series of X-rays of each specimen of 
lead respectively. To get the grazing angles of incidence to substitute in the 
above formula for the wave-length we measured from the centres of the drops 
to the zero, 291° 55' 40'', the value of which has been corrected for eccentricity. 
The values of the wave-lengths, etc., have been collected together in the 
following table. 
ABSORBING SCREEN 
ATOMIC WEIGHT 
X X 10? CM. 
X X 108 CM. 
X X 103 CM. 
Ordinary lead 
207.20 
0.9485 
0.8128 
07.806 
206.08 
0.9489 
0.8129 
0.7810 
The grazing angles of incidence can be estimated to within about 30" of 
arc, which means that the wave-lengths are correct to within about 0.1%. 
Corresponding wave-lengths for the two specimens in the above table do not 
differ from each other by as much as 0.1%, and, therefore, the critical ab- 
sorption wave lengths of the isotopes of lead are identical to within the limits 
of error of the experiments. 
The magnitude of the characteristic absorption can be estimated from the 
drops in the curve. These depend, of course, upon the thickness of the ab- 
sorbing layerj and its measurement is very much less accurate than that of 
the wave-length. It appears, however, that the relative change of absorption 
at the three critical wave-lengths is about the same for each isotope of lead. 
