Vol. 6, 1920 PHYSICS: DUANE AND PATTERSON 509 
CHARACTERISTIC ABSORPTION OF X-RAYS: L SERIES 
By W1I.UAM DUANE AND R. A. PATTERSON 
Department of Physics, Harvard University 
Communicated August 2, 1902 
Introduction. — Critical absorption wave-lengths associated v^ith the 
L series of X-rays have been measured by E. Wagner^ and M. de Broglie.^ 
These scientists have investigated the spectra of ten of the chem- 
ical elements from tungsten to uranium, both inclusive. Each employed a 
photographic X-ray spectrometer w^ith a rocksalt crystal. M. de Broglie 
found two critical absorption v^ave-lengths characteristic of each chem- 
ical element except, in the case of mercury, for which he gives only one. 
For bismuth, thorium and uranium he found a third, weak band. 
In the investigation reported in this note we employed an ionization 
spectrometer with a calcite crystal. We found three critical absorption 
wave-lengths characteristic of each of the nine elements examined. 
. A brief discussion of the bearing of this new data on certain empirical 
laws and on recent theories of the mechanism of X-radiation may be found 
in the concluding paragraphs. 
Apparatus and Method of Measurement. — A detailed description of the 
ionization spectrometer and the method of using it so as to obviate certain 
errors of measurement has been given in previous articles.^ These arti- 
cles also describe the plant for generating and controlling the X-radiation. 
Two X-ray tubes have been employed, each equipped with a Coolidge 
cathode and with a thin glass window blown in a side arm. One of these 
tubes contained a tungsten and the other a molybdenum target. The 
current exciting the tubes came from a high tension storage battery. 
The ionization chamber contained ethyl-bromide or methyl-iodide, and 
sometimes a mixture of both. 
In measuring a critical absorption wave-length we place a thin sheet of 
the chemical element to be investigated (or of a salt containing it) in the 
path of the beam of rays between the X-ray tube and the first slit of the 
spectrometer. We then measure the ionization currents for a series of 
positions of the crystal corresponding to wave-lengths in the neighbor- 
hood of the critical values. From these measurements we platt curves 
representing the ionization current as a function of the crystal table angle 
(see figs. 1 and 2). At a critical absorption point a sharp drop appears 
on the curve, indicating that X-rays of shorter wave-length than the 
critical value are absorbed by the chemical element to a greater extent 
than X-rays of longer wave-length. For the two strongest absorption 
bands characteristic of each element we have obtained the curves on both 
sides of the crystal table zero. The angular distance between the mid- 
points on two corresponding drops gives us twice the glancing angle, 6, 
