Principle of the Quantum Theory. 1115 



harmonic vibrations of new type and of amplitudes propor- 

 tional to the intensity of the external forces. To call such 

 an effect, as is often done, a " breaking through " of the 

 principle of selection is a terminology which would not 

 seem to be quite adequate in view of the character of the 

 theoretical interpretation which can be given of the pheno- 

 menon under consideration. 



In the new experiments published in the note, mentioned 

 in the beginning of this letter, precautions were taken to 

 screen the radiating atoms from the effect of external electric 

 forces due to the potential applied to the discharge tube. 

 In spite of these precautions it was found that under certain 

 conditions the spectra observed included, besides the usual 

 arc lines of potassium, certain lines corresponding to combi- 

 nations between two terms, for which n% differs by two 

 units, and which correspond to the new series mentioned 

 above, which appear when the radiating atoms are exposed 

 to intense electric fields. This observation is by the authors 

 described as a significant exception to the principle of selec- 

 tion, since the appearance of the lines could evidently not 

 be caused by external fields of the same type as claimed 

 for the production of the ordinary Stark effect. From a 

 closer consideration of the experimental conditions it would 

 appear, however, that the observations published rather 

 support than disprove the theory. First of all it was found 

 that the lines in question appeared only if very dense 

 currents were sent through the apparatus, while these lines 

 did not appear when a discharge of less current density was 

 passed through the tube. Thus under the circumstances, 

 where the condition of non-disturbance of the atoms by 

 external agencies, claimed by the theory, was satisfied to 

 the highest degree, no exception from the simple selection 

 rule was observed, indicating that the presence of the 

 new lines for higher current densities was caused by some 

 agency influencing the usual motion of the electrons in the 

 atoms. Looking for some such effect, it would appear that, 

 just due to the screening from external forces, the experi- 

 mental arrangement described would be especially favour- 

 able for the accumulation of ions in the region of the 

 discharge tube used for the observation of the spectrum ; 

 an effect to which already the authors themselves incidentally 

 refer as a possible cause of the origin of the new lines. 

 Without closer information about the dimensions of the 

 apparatus and the details of construction than that given 

 in the note, it is not possible to form an accurate estimate 

 of the density of ions which may have been present 



