408 Dr. N. Bohr on the Quantum Theory of 



theory if the spectra are emitted by atoms which have lost 

 two electrons and are regaining one of them. In this case, 

 the outer electron will rotate round a system o£ double 

 charge, and we must assume that in the stationary states it 

 will have configurations approximately the same as an elec- 

 tron rotating round a helium nucleus. This view seems in 

 conformity with the general evidence as to the conditions of 

 the excitation of the ordinary spectra and the spectra of 

 enhanced lines. From Fowler's results, it will appear that 

 the helium spectrum given by (3) for N — 2 has exactly the 

 same relation to the spectra of enhanced lines of other 

 elements as the hydrogen spectrum has to the ordinary 

 spectra. It may be expected that it will be possible to 

 observe spectra of a new class corresponding to a loss of 

 3 electrons from the atom, and in which the Rydberg con- 

 stant K is replaced by 9K. No definite evidence, however, 

 has so far been obtained of the existence of such spectra *. 



Additional evidence of the essential validity of the inter- 

 pretation of formula (13) seems also to be derived from the 

 result of Stark's experiments on the effect of electric fields 

 on spectral lines. For other spectra, this effect is even more 

 complex than for the hydrogen spectrum, in some cases 

 not only are a great number of components observed, but the 

 components are generally not symmetrical with regard to 

 the original line, and their distance apart varies from line 

 to line in the same series in a far more irregular way than 

 for the hydrogen lines f- Without attempting to account 

 in detail for any of the electrical effects observed, we shall 

 see that a simple interpretation can be given of the general 

 way in which the magnitude of the effect varies from series 

 to series. 



In the theory of the electrical effect on the hydrogen 

 spectrum given in the former section, it was. supposed that 

 this effect was due to an alteration of the energy of the 

 systems in the external field, and that this alteration was 

 intimately connected with a considerable deformation of 

 the orbit of the electron. The possibility of this de- 

 formation is due to the fact that without the external 

 field every elliptical orbit of the electron in the hydrogen 

 atom is stationary. This condition will only be strictly 

 satisfied if the forces which act upon the electron vary 

 exactly as the inverse square of the distance from the 

 nucleus, but this will not be the case for the outer electron 

 in an atom containing more than one electron. It was pointed 



* Fowler, he. cit. p. 262, see also II. p. 490. 

 t Stark, he. cit. pp. 67-75. 



