1114 Prof. N. Bohr on the Selection 



the fact that it is possible in certain cases to exclude tran- 

 sitions between stationary states from the condition of con- 

 servation of angular momentum during the radiation process. 

 As regards the problem under consideration this leads to the 

 conclusion, that the quantum number n z , which, as mentioned, 

 is directly related to the resultant angular momentum of the 

 atom, by a transition cannot vary by more than one unit, 

 while no direct information as regards a limitation of the 

 quantum numbers % and n 2 can be obtained from this 

 argument. 



As regards the comparison of these conclusions with 

 experiments, our insight into the origin of the complex 

 structure of spectral lines is at present hardly suffi- 

 ciently developed to provide a definite test as regards the 

 detailed interpretation of the quantum number n z and there- 

 fore of the rules of selection as far as they can be based 

 entirely on considerations of conservation of angular 

 momentum. At present the main problem, with which also 

 the experiments in the above-mentioned note deal, is the 

 test of the conclusions as regards the variation of the 

 quantum numbers n v and n 2 , on which in first approxima- 

 tion the spectral terms depend. Now previous experimental 

 evidence as regards excitation of series spectra has seemed 

 convincingly to support the conclusion drawn from the 

 principle of correspondence, that under circumstances where 

 the emitting atoms are not influenced by external agencies, 

 only such spectral lines can appear as correspond to a 

 combination of terms for which n 2 differs by one unit. 

 This may be considered as a very important result, because 

 we may say that the quantum theory, which for the first 

 time has offered a simple interpretation of the fundamental 

 principle of combination of spectral lines, at the same time 

 has so to say removed the mystery which has hitherto 

 adhered to the application of this principle on account of 

 the apparent capriciousness of the appearance of predicted 

 combination lines. Especially attention may be drawn to 

 the simple interpretation which the quantum theory offers 

 of the appearance observed by Stark and his collaborators of 

 certain new series of lines, which do not appear under 

 ordinary circumstances, but which are excited when the 

 emitting atoms are subject to intense external electric fields. 

 In fact, on the correspondence principle this is immediately 

 explained from an examination of the perturbations in the 

 motion of the outer electron, which give rise to the appear- 

 ance in this motion — besides the vibration already present 

 in a simple central orbit — of a number of constituent 



