Principle of the Quantum Theory. 1113 



refers to a certain condition concerning the radial motion 

 of the electron, n 2 fixes the angular momentum of the 

 electron round the centre of the orbit through the condition 

 that this angular momentum is equal to n 2 h\2ir. These 

 numbers are assumed to be related to the spectral terms in 

 such a way that n x increases by one unit when, within each 

 series of terms, we proceed from one member to the next, 

 while n 2 remains constant within each series of terms and 

 increases by one unit when we pass from the S-terms to the 

 P-terms, and from the P-terms to the D-terms, and so on. 

 This classification of the terms refers, however, only to the 

 structure of the arc spectra in large features. In order to 

 account for the complex structure of the lines (doublets, 

 triplets) a higher degree of complexity of the multitude of 

 stationary states is claimed. This is assumed to arise from 

 a complication of the motion of the outer electron due to a 

 small departure from central symmetry of the inner system, 

 which causes the plane of the orbit of the outer electron to 

 undergo a slow precession round an axis coinciding with the 

 axis of angular momentum of the atom. Due to this compli- 

 cation of the motion there will in the fixation of the stationary 

 states appear a third quantum number ?* 3 , which fixes the 

 orientation of the plane of the outer electron relative to 

 the axis of the inner system through the condition that the 

 resultant angular momentum of the atom is equal to ?? 3 /</27r. 

 This third quantum number is related to the complexit} r of 

 the multitude of spectral terms in such a way that the 

 components of a set of complex terms corresponding to the 

 same values of iii and n 2 are distinguished by different 

 values of n z . 



Now the so-called principle of selection originates from 

 considerations dealing with the limitation of the possibility 

 of transition between stationary states. Such considerations 

 are based on two entirely different types of arguments. 

 One argument rests upon the so-called principle of corre- 

 spondence according to which the possibility of a transition 

 between two stationary states, giving rise to the emission 

 or a train of harmonic waves, is sought in the presence in 

 the motion of the atom of a certain "corresponding" con- 

 stituent harmonic vibration. For stationary states of the 

 atom of the type described above, this argument leads to 

 the conclusion that, at the same time as no limitation is 

 imposed on the variation of the quantum number « x , the 

 number n 2 must by a transition always change by one unit, 

 while the number n & may either change by one unit or 

 remain unchanged. Another argument is obtained from 



