512 Dr. N. Bohr on the Effect of 



atom in which the configuration of the inner electrons is very 

 nearly the same for all states in one series, while the con- 

 figuration of the outer electron changes from state to state 

 in the series approximately in the same way as in the 

 hydrogen atom. 



It will appear that these considerations offer a possible 

 simple explanation of the appearance of the Rydberg con- 

 stant in the formula for the spectral series of every element. 

 In this connexion, however, it may be noticed that on this 

 point of view the Rydberg constant is not exactly the same 

 for every element, since the expression (8) for K depends 

 on the mass of the central nucleus. The correction due to 

 the finite value of M is very small for elements of high 

 atomic weight, but is comparatively large for hydrogen. 

 It may therefore not be permissible to calculate the Rydberg 

 constant directly from the hydrogen spectrum. Instead of 

 the value 109675 generally assumed, the theoretical value 

 for a heavy atom is 109735. 



§ 2. The Effect of an Electric Field. 



As mentioned above, J. Stark has recently discovered 

 that the presence of an external electric field produces a 

 characteristic effect on the line-spectrum of an element. 

 The effect was observed for hydrogen and helium. By 

 spectroscopic observation in a direction perpendicular to the 

 field, each of the lines of the hydrogen spectrum was broken 

 up into five homogeneous components situated very nearly 

 symmetrically with regard to the original line. The three 

 inner components were of feeble intensity and polarized 

 with electric vector perpendicular to the field, while the two 

 outer stronger components were polarized with electric vector 

 parallel to the field. The distance between the components 

 was found to be proportional to the electric force within the 

 limits of experimental errors. With a field of 13,000 volt 

 per cm. the observed difference in the wave-length of the 

 two outer components was 3*G x 10 -8 cm. and 5*2 x 10~ s cm. 

 for Hp and TI y respectively. For both systems of lines 

 emitted by helium, Stark observed an effect on the lines of 

 the Diffuse series which was of the same order of magnitude 

 as that observed for the hydrogen lines, but of a different 

 type. Thus the components were situated unsymmetrically 

 with regard to the original line, and were also not polarized 

 relative to the field. The effect of the field on the lines of 

 the Principal series and the Sharp series was very small and 

 hardly distinguishable. 



On the theory of this paper the effect of an external field 



