522 Dr. N. Bohr on the Effect of 



nucleus, P will be zero and the perturbing forces will be 

 given by the higher terms in (25). 



Let us now assume that for a certain series of stationary 

 states P is different from zero. According to our con- 

 siderations, the major-axis of the orbit of the outer electron 

 is approximately equal to that in the stationary states of the 

 hydrogen atom. The major-axis will therefore be approxi- 

 mately proportional to n 2 > Accordingly the quantity corre- 

 sponding to E in the equation (19) and due to the second 

 term in (25) will vary approximately as n~ G . The difference 

 in the energy of the two stationary states corresponding to 

 (19) may therefore be expected to vary approximately as ?i~ 4 . 

 This corresponds to the variation observed for the distances 

 between the components of the double lines in the spectra of 

 the alkali metals. 



The visible spectra of the alkali metals consist of three 

 series of double lines. The difference in frequency of the 

 components of the lines of the Sharp series and the Diffuse 

 series is the same for every line. For the Principal series the 

 difference diminishes rapidly with the number of the line in 

 the series, the difference being approximately proportional 

 to the inverse fourth power of this number. It will appear 

 that this spectrum can be interpreted on the assumption of 

 three series of stationary states of the atom, corresponding 

 to different configurations of the inner electrons ; viz.: two 

 single series T. and IT., and a double series III. representing 

 for every n two stationary states for which the difference in 

 energy varies in proportion to n~ 4 . The Principal series of 

 doublets corresponds to a transition from a pair of the 

 states III. to the first state of I., while the Sharp and Diffuse 

 series correspond to transitions respectively from states I. 

 and II. to the first pair of state III. 



I shall not here try to develop these considerations in 

 further detail, but confine myself to show that the view 

 adopted seems to indicate a possible explanation of the results 

 of the experiments by Paschen and Back on the effect of a 

 magnetic field on spectral lines of complicated structure. 

 The characteristic result of these experiments is the great 

 difference between the effect of a weak and a strong mag- 

 netic field. In the presence of a weak magnetic field the 

 components of a double line are resolved in a complicated 

 way. If the field increases, the distances between the sub- 

 components at first increase regularly with the strength of 

 the field. When, however, the distances are of the same 

 order of magnitude as the distance between the components 

 of the original double line, the aspect of the system of lines 



