75 



359 



AU this is in general agreement with the observations as it will be seen from 

 a detailed discussion of the fine structures observed. Before entering on such a 

 discussion, however, it will be necessary to consider in a certain detail the in- 

 fluence which small perturbing electric forces will have on the fine structure 

 of the hydrogen lines and helium lines under consideration. That such an 

 influence must beforehand be expected to be very considerable may for instance 

 be seen from the circumstance that rather small electric fields will be sufficient to 

 disturb entirely the character of the fine structure and to give rise to a regular 

 Stark effect. Thus, in the case of //^, an electric field of 1000 ^'«'t cm would already 

 give rise to a Stark effect for which the distance of the outer parallel components 

 is equal to nearly two times the width of the original fine structure doublet of H,^. 

 From a mechanical point of view the easiness with which a fine structure is 

 disturbed by a small external electric force is interpreted by observing that the 

 deviation of the orbit of the electron from a purely periodic orbit due to the in- 

 fluence of the relativity modifications is extremely small, so that already a com- 

 paratively small electric force will produce alterations in the orbit which are of 

 the same order of magnitude. As it will appear in the following sections a dis- 

 cussion of the effect of a weak electric field is of essential importance in order 

 to obtain a theoretical understanding of the typical manner in which, on many 

 of Paschen's spectrograms, the intensity distribution of the different components 

 deviates from the simple intensity distribution to be expected from the preceding 

 considerations in this section. 



//. Effect of a weak electric fietd on the fuie strnclnre of the liijdroijen lines. 



A general discussion of the effect which an electric field must be expected to 

 have on the fine structure of a hydrogen line when its intensity increases from 

 zero, so that the fine structure is gradually transmuted into an ordinary Stark etTect 

 in which the relativity modifications play only a secondary part, will, as mentioned 

 in the beginning of § 4, be given in a later paper. Here we will only discuss the 

 cflect of an electric field, the intensity of which is so small that its influence is still 

 small compared to that of the relativity modifications. 



The character of the influence of a small external constant field of force on 

 llie spectrum of an atomic system has been treated by Bohr in the first and in the 

 second Part of his often mentioned paper. As regards the frequencies of the spectral 

 lines this effect may be directly found by means of (1) as soon as it is possible to 

 fix the energy in the stationary states of the perturbed system. This constitutes a 

 problem which in general may be solved if the deviations of the mechanical motion 

 of the perturbed system from the motion in the undisturbed system are at any 

 moment very small, and for its treatment the fundamental principle of the mechani- 

 cal transformability of the stationary slates, which has been introduced in the 



4Ü* 



