Introduction. 



In the quantum theory of line spectra it is assumed that the laws of electro- 

 dynamics cannot be applied to atomic systems, and the assumption is made thai 

 an atomic system can exist without emitting radiation in a number of states, which 

 are called the "stationary slates" of the system, and thai a process ol emission or 

 absorption of energy can only lake place by a complete transition between two 

 such stales. Further it is assumed that Ihe radiation emitted or absorbed during 

 such a transition is unifrequentic, and that its frequency is given by 



u=j^(E'-E"), (1) 



where E' and E" are the values of the energy in the two states, and where h is 

 Planck's constant. As well known Bohr was able, on the basis of these assump- 

 tions, to account in a convincing way for the frequencies of the lines of the series 

 spectrum of hydrogen and for some main features of the series spectra of other 

 elements. In the course of the last years the ([uantum theory of line spectra has 

 been developed considerably, due to the work of Sommehfeld, Epstein, Schwarz- 

 schild and others, who, by extending Bohr's original theory, were able to explain, 

 as regards the frequencies of the components, the characteristic fhie structure of 

 the hydrogen lines and the etîect which strong external electric or magnetic fields 

 have on these lines. Now Bohr') has shown in a recent paper, which contains a 

 general exposure of the principles of the quantum theory of line spectra, that it is 

 not only possible to get information as regards the frequencies of spectral lines, 

 bul that at the present state of the theory we are also able to draw some con- 

 clusions regarding the polarisation and intensities with which these lines 

 appear, l)y considering the amplitudes of the harmonic vibrations in which the 

 motion of the particles in an atomic system may be resolved. On professor Bohr's 

 proposal 1 have undertaken in the present paper to treat in detail the application 

 of his ideas to the problem of Ihe intensity of spectral lines in the special case of 



') N. Bohr, On the (Juaiitimi l luory of Line Spectra. D K^l. Danske Vidciisk. Selsk. Skr. iiatur- 

 vidensk. og inathem. Afd.. 8. Kække IV. 1. 1918. This paper will in the follo\vinj< he referred to as: 

 N. Bohr, loe. cit. 



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