Vol. 7, 1921 
PHYSICS: R. A. MILLIKA N 
289 
THE FURTHER EXTENSION OF THE ULTRA-VIOLET 
SPECTRUM AND THE PROGRESSION WITH ATOMIC NUMBER 
OF THE SPECTRA OF LIGHT ELEMENTS 
By R. A. MiivUKAN 
Ryerson Physical Laboratory, University of Chicago 
Read before the Academy April 26, 1920 
The chief purpose of this investigation, outlined and begun in 1916 1 
and briefly reported upon last year, 2 has been to explore in the extreme 
ultra-violet the radiations which can be emitted by the second ring or 
shell of electrons in the atoms of atomic number from 2 to 13 (helium 
to aluminium). 3 
The results obtained to date may be very briefly summarized as follows : 3 
1. The ultra-violet spectrum has been photographed and its wave- 
lengths determined down to X = 136.6 Angstroms in the case of aluminium 
and down to X = 149.5 Angstroms in the case of copper. There is thus 
a gap represented by a factor of but 10 between the shortest measured 
ultra-violet waves and the longest X-rays measured by the method of 
crystal-spectrometry which stops at 13.3 Angstroms. Fortunately, how- 
ever, in the ultra-violet region, which has been already opened up and ex- 
plored, the most interesting and the most important of the hitherto inaccessible 
radiations are found; — thus 
2. The L a lines of Al, Mg, and Na have been photographed and located 
at 144.3 A, 232.2 A, and 372.2 A, respectively. These wave-lengths are 
all fairly accurately on the Mosely line connecting La. frequencies and 
atomic number (see fig. 1). It has thus been definitely proved that the L 
series continues with its main characteristics unchanged throughout the: 
whole range of atomic numbers from Ur (92) to Ne (10). The linear pro- 
gression thus revealed clear down to neon could be very roughly inferred 
from the beautiful measurements of Hjalmar, 4 which gave the K a and 
Kp lines of the elements down to sodium. For the Kossel relation between 
the frequencies of the K and L series, namely, Kp — K a = L a , although 
known to be quite inexact, was presumably sufficiently near to the truth 
to enable the order of magnitude of the La. wave-lengths to be predicted. 
Kossel has already made this use of Hjalmar's data. 5 His values of 
L a computed for Al, Mg and Na in this way are actually about 20%, 
too low. 
3. It has been found that the aluminium atom {atomic number 13) when 
excited by these condensed sparks in vacuo emits no radiations whatever of 
wave-length between 144.3 A and about 1200 A where its M spectrum, that 
due to its three outer electrons, begins and extends with considerable 
complexity into the visible. This shows that optical spectra are quite like 
