520 
PHYSICS: DUANE AND PATTERSON Proc. N. A. S. 
the K series of tungsten and that of its y Hne amounts to about one-half 
of one per cent, the 7 Hne having the longer wave-length. In the case 
of each element the wave-lengths were measured under exactly the same 
experimental conditions, so that there can be no doubt but that the 
critical absorption wave-length in the K series is shorter than the wave- 
length of the 7 line by an amount that considerably exceeds the errors of 
experiment. 
Turning to the L series we find that the third critical absorption wave- 
length. Las, appears to be shorter than that of the line of shortest wave- 
length in the L series, namely, 74.^ The authors^ have found the 
following values for these wave-lengths, Las = 1 .024 ± 3 and 74 = 1.0261 =t 6, 
in the spectrum of tungsten. In the other two groups, however, the 
critical absorption wave-lengths are longer than those of the shortest 
wave-length lines respectively. In the tungsten spectrum the authors^'^ 
found for the wave-lengths in the first group Lai = 1.2136 ±1 and /55 = 
1.2040 ±7, and in the second Las =1.0726 ±5 and 72= 1.0655 ±4. In 
each case the difference between the two wave-lengths considerably ex- 
ceeds the estimated errors of experiment. 
As the relative position of these emission and absorption lines appears 
to be a matter of considerable theoretical importance the authors have 
repeated their experiments in a slightly different form. To make doubly 
sure of the facts they have taken the readings so as to show the position 
of the critical absorption and those of the emission lines on each side of it 
on one and the same curve. The experimental conditions were exactly 
the same as those in the experiments described in the article referred to,^ 
except that the X-ray tube was turned so that the rays left the target 
making a slightly smaller angle with its surface than in the earlier experi- 
ments. This increased the absorption of the rays by the target itself, 
and made the absorption drop in the curves more prominent. 
The curve in figure 1 represents the intensity of the radiation (meas- 
ured by the currents in the X-ray spectrometer's ionization chamber) as 
a function of the crystal table angles. The drop in the curve marked 
ai corresponds to the critical absorption belonging to the first group of 
L series emission lines. The absorption of its own rays by the tungsten 
target alone produced this drop, as no absorbing screen was introduced in 
the path of the rays in this experiment. The tall peak on the curve 
represents the relatively strong emission line /Js- Its separation from the 
absorption drop corresponds very well with the wave-length interval be- 
tween the two, calculated from the data obtained in the earlier experi- 
ments, namely 0.0283 =±=2. The small hump in the curve on the other 
side of the absorption drop represents the emission line, /Ss. 
This experiment proves conclusively that the critical absorption lies 
between the emission lines ^2 and (3^, in other words it has a longer wave- 
length than that of 
