290 
PHYSICS: R. A. MILLIKAN 
Proc. N. A. S. 
X-ray spectra in that large gaps occur between the frequencies due to the 
electrons in successive rings or shells. If we could neglect the influence of 
the negative electrons upon one another we could compute the relative 
diameters of these shells, for they would be inversely proportional to the 
limiting frequencies, i.e., in this case in about the ratio 1 to 9; but such 
computations are of little value save as the roughest sort of indices. The 
chief lines below 2000 A, due to the three outer electrons of the Al atom 
have the wave-lengths 1379.7 A, 1384.5 A, 1605.9 A, 1612.0 A, 1671.0 A, 
1854.7 A and 1862.7 A. 
Magnesium shows a behavior quite like that of aluminium in that we 
find a complete blank between its L a line at 232.2 A and the lines due 
to its two outer electrons whose radiations begin on our plates on the 
short wave-length side at about 1700 A. The chief radiations below 
2000 A, arising from these two outer electrons of the magnesium atom, 
have the wave-lengths 1735.2 A, 1737.9 A, 1751.0 A, and 1753. 7 A. 
Also, sodium, quite consistently with the foregoing, is found to emit 
no radiations whatever between its L lines, the longest of which is at 376.5 A 
and the lines due to its single M electron which have their convergence 
wave-length at 2412.63 A, and reach their maximum intensity in the fa- 
miliar sodium doublet at 5890 A and 5896 A. 
4. Coming now to the group of atoms below neon, atomic number 10, 
the spectra due to the electrons in the incomplete second or L-ring in the 
case of these atoms are completely unpredictable from any theory that 
we now have but they have been experimentally obtained. For reasons 
which will appear they will not be considered in the order of atomic number. 
a. The spectrum due to the six L-ring electrons of the oxygen atom 
(atomic number 8) begins upon our plates at about 230 A and extends 
with much complexity and strength up to 834.0 A where the strongest 
oxygen line, which will be arbitrarily called its L a line, is found. Above 
834.0 A the oxygen lines are few in number and relatively faint. 
Since the K a line of O can be computed with much certainty to be 
at 23.68 A, the ratio of the K frequency to the L frequency for the oxygen 
atom is about 35. It will be recalled that this ratio is only about 7 in 
the case of atoms of high atomic number and that it slowly increases 
with decreasing atomic number, reaching the values 17.2, 23.4 and 31.1 
in the cases of Al, Mg, and Na, respectively (see above). The strongest 
of the oxygen lines have the following wave-lengths: 321.2, 374.3, 507.8, 
525.7, 554.2, 599.5, 610.1, 616.7, 625.2, 629.6, 644.0, 703.1, 718.5 and 
834.0. 
The oxygen lines have been identified because they appear as impuri- 
ties in all easily oxidizable metals, NrU, Mg, Al, Zn, Fe, etc., The 
key to the oxygen spectrum was furnished by the discovery that chemi- 
cally pure aluminium and magnesium showed the extraordinary property 
