6l2 
PHYSICS: DUANE, FRICKE AND STENSTROM Proc. N. A. S. 
as satisfactory as the others. The generating plant had to be pushed to 
the Hmit of its capacity, and the fluctuations in the X-ray output were 
unusually large. 
Our values of the wave-lengths are uniformly larger than those ob- 
tained by photographic methods. On the average these differences amount 
to between one and two per cent. The measurements by Siegbahn and 
Jonsson were made with an interesting spectrometer specially designed 
to eliminate errors due to the penetration of the X-rays into the reflecting 
crystal, etc. In an ordinary spectronometer, if the grazing angle 6, is 
determined by the position of the reflected beam of X-rays these errors 
may be either positive or negative, according to the position of the axis 
of rotation of the crystal with reference to what we have called the elective 
reflecting plane. Further, they may increase or decrease with the wave- 
length of the rays, and in limiting cases the size of the crystal would have 
an effect on them. Our method of using the spectrometer eliminates 
these errbrs, for we determine the grazing angle, 6, from the angle thi^ugh 
which the crystal turns and not by the position of the reflected beam. 
The differences, therefore, between our values and those given by Sieg- 
bahn and Jonsson cannot be ascribed to errors of this kind. Possibly 
they may be due to differences in the manner of interpreting the experi- 
mental observations. If, for instance, measurements on a photographic 
plate were made from the point where the plate begins to get dark, the cor- 
responding value of the critical absorption wave-length would be shorter 
than that given by our method, for we measure between points half way 
up the steep drops. The point where the photographic plate begins to 
darken corresponds to the bottom of a drop on our curves. 
The wave-lengths contained in table 1 complete the series of measure- 
ments of the K critical absorption of the chemical elements that we have 
been making in our X-ray laboratory during the past few years. We 
now have values of the K critical absorption wave-lengths for most of 
the chemical elements from manganese (atomic number 25) to uranium 
(atomic number 92) both inclusive. These wave-lengths have been meas- 
ured by means of the same ionization spectrometer and with the same 
calcite crystal, and are therefore comparable with each other. 
1 Duane and Hu, Physic. Rev., Ithaca, N. Y., Oct., 1918 (369). 
2 Duane and Stenstrom, Washington, Nat. Acad. Proc, Aug., 1920. 
3 Duane and Shimizu, Physic. Rev., Dec, 1919 (522). 
4 Blake and Duane, Ibid., Dec, 1917, 697; and Duane and Hu, Ibid., Dec, 1919 (516). 
^ De Broglie, /. Physique, Paris, May-June, 1916 (161). 
® Siegbahn and Jonsson, Physik. Zeits., 1919 (251). 
7 Duane and Hunt, Physic. Rev., Aug., 1915 (166). 
s Stenstrom, Doctor's Dissertation, Lund., 1919. 
