518 Dr. N. Bohr on the Effect of 



on the other two series. This is in agreement with Stark's 

 experiments *. 



On the present point of view a line of the Diffuse series 

 of the helium spectrum corresponds to a transition between 

 two stationary states, one of which is affected by the presence 

 of an electric field, and the other not: while for the hydrogen 

 lines both states were assumed to be affected by the field. 

 This circumstance may afford an explanation of the fact 

 observed by Stark, viz. that the components of the helium 

 lines were not polarized relatively to the field like the 

 hydrogen lines. 



§ 3. The Effect of a Magnetic Field. 



An effect of a magnetic field on the line-spectrum of an 

 element was discovered by Zeeman in 1896. By spectro- 

 scopic observation in a direction perpendicular to the field 

 the lines are resolved, in the simplest case, into symmetrical 

 triplets of which tne central components have the same 

 position as the original line and are polarized with electric 

 vector parallel to the field, while the outer components are 

 polarized with electric vector perpendicular to the magnetic 

 field. 



As is well known, Lorentz succeeded in explaining this 

 result on the basis of the classical electron theory. According 

 to his calculation, which was found to agree with Zeeman's 

 observation within the limit of experimental error, the differ- 

 ence in frequency between the outer and the inner com- 

 ponents is the same for every spectral line, and equal to 



t =4c4 h > < 23 > 



where H is the magnetic force and c the velocity of light. 



Later more complicated types of magnetic effect on spectral 

 lines have been observed. In most cases, however, simple 

 numerical relations are found to exist between the distance f 

 of the components observed and that calculated by Lorentz. 

 Further, the recent experiments by Paschen and Back X on 

 the magnetic effect on double lines, which will be mentioned 

 in the next section, indicate that the complicated types of 

 Zeeman effect are intimately connected with complication 



* Since the value of 4> r (n) differs widely from unity for all series of 

 lines in the spectra of the heavier elements, it is to be expected that 

 the electric effect should be very small, or undetectable for such 

 elements. 



f See 0. Eunge, Phys. Zeitschr. viii. p. 232 (1907). 



+ Ann. d. Phys. xxxix. p. 8U7 (1912), xl. p. 900 (1913). 



