Vol. 6, 1920 PHYSICS: DUANE AND STENSTROM 485 
that the Lai, hoi2 and L/3i Hnes are due to electrons falHng from the Mi 
and M2 orbits into L orbits, whereas we inferred above that the KjS hne 
was mainly due to electrons falling from the M3 orbit into the K orbit. 
A rough quantitative test for this explanation may be obtained as follows : 
From the above three equations and the three equations representing the 
transfers of electrons producing the Ka and K/3 lines, namely 
Ka — Lai = Kai, Ka — La2 = Kq!2 and Ka — Mas = K/S, 
we get the equations 
Kai + Lai — K/3 = Mas — Mai and Kq;2 + L/?i — K/3 = Mas — Ma2. 
The left-hand members of these equations have the values 0.043 and 
0.039, respectively, for tungsten. The right-hand members, which are the 
differences between quantities that we have estimated by extrapolation, 
have the values 0.030 and 0.025, respectively. These small differences are of 
the same order of magnitude, which indicates that the explanation is approxi- 
mately correct. We must remember, however, that the M series is prob- 
ably more complicated than we have supposed. There are reasons for 
believing, for instance, that another critical absorption exists, of slightly 
higher frequency than Mas. 
The ionization spectrometer furnishes us a method of approximately 
estimating the relative intensity of the lines in X-ray spectra. At pres- 
ent, however, we are not able to correct for the change with wave-length 
in the absorption of the rays by the target, the glass walls of the X-ray 
tube and ionization chamber, the reflecting crystal, etc. Nor do we know 
exactly how the coefficient of reflection of the crystal varies with the wave- 
length. It is not safe, therefore, to compare spectral lines that differ 
very much from each other in wave-length. 
By estimating the heights of the peaks corresponding to the lines of 
the K series in the spectra of the first, second, third and fourth orders 
we have arrived at the following numbers representing approximately their 
relative intensities: 
Line as 02 ai ^ > 
Intensity 4 50 100 35 15 
It is interesting to note that, if we apply Sommerf eld's theory of the 
relative intensity of lines (Atombau und Spektrallinien, chapter 6) to 
the oil and lines, we get either 3 : 2, or 2 : 1, as the ratio of their in- 
tensities. Our experiments favor the latter ratio. 
In an address delivered at the St. Louis meeting of the American As- 
sociation for the Advancement of Science, last December (see Science, 
May 21, 1920, p. 505) one of us presented a set of calculations of the K 
critical absorption frequencies for a number of chemical elements. The 
calculations were based on Bohr's theory, with the additional assumption 
that the electrons were distributed among the orbits in much the same way 
