JOURNAL 
OF THE 
WASHINGTON ACADEMY OF SCIENCES 
VoL. 13 JUNE 19, 1923 No. 12 
SPECTROSCOPY .—Regularities in the arc spectrum of tron.! F. M. 
Watters, Jr., Bureau of Standards. 
According to the Bohr theory a particular line of the so-called are 
spectrum of an element is emitted when the energy of a normal atom 
changes in a manner governed by quantum conditions. Thus a 
spectral line of frequency » corresponds to an energy change of 
hy =E; —E; where E; and E; are the initial and final states of the 
atom. <A multitude of possible states or energy levels exists for each 
atom and different spectral lines result from different combinations 
of these. Further, the different sets of levels are usually polyfold, 
that is, each so-called state may really consist of two or more slightly 
different conditions of energy. Intercombinations of these multiple 
levels give rise to groups of spectral lines which increase in complexity 
as the multiplicity of the levels involved in their production increases. 
The frequency differences of the lines are proportional to the energy 
differences of the associated levels, and when one multiple set of levels 
enters into combination with two or more different sets, the corre- 
sponding groups of lines will repeat the wave number differences char- 
acterizing this set of levels. These repeated differences have been 
shown by interferometer measurements? to be strictly constant, and 
they are, therefore, a positive criterion for the analysis of spectra. 
In addition to the well-known spectral series and intercombinations 
consisting of singlets, doublets and triplets, more complex groups 
of lines were discovered by Popow* and very recently still more 
complicated groups have been found by Catalan‘ in the spectra of 
manganese and chromium and by Kiess® in the are spectrum of 
molybdenum. For these complex groups Catalan coined the work 
“multiplets.” 
1 Received May 23, 1923. Published by permission of the Director, Bureau of 
Standards. Communicated by Dr. W. F. Meggers. 
2 Bureau of Standards Scientific Papers, 329, 414 and 441. 
3 Annalen der Physik 45: 147. 1914. 
4 Phil. Trans. A, 223: 127. 1922, Fisica y Quimica 21 p. 84, 1923. 
5 Bureau of Standards Scientific Paper, in press. 
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