124 



Mr. E. C Kemble on the Probable 



The two sj'stems of series in the arc spectrum of helium are 

 explained by Lande * as due to two different sets of outer 

 orbits in planes differently oriented with respect to the 

 plane of the orbit of the inner electron. His computations 

 indicate that the doublet system of spectrum lines originates 

 in a set of outer orbits coplanar with the path of the inner 

 electron, while the single-line system comes from a set 

 of outer orbits whose planes are inclined to that of the inner 

 orbit. 



Fig. 1 shows diagram atically the energies and series 

 relationships of the various stationary states of the helium 

 atom. 



Fig. 1. 



Negctive Enerdiefc on Wqye'Number SpalexlO [(Term Values) 



2 



23 



30 



35 



HO 



15 



The normal state of an atom is the stationary state with 

 the least energy. Its energy (negative) is measured by the 

 ionization potential of the atom, and also by the frequency 

 of the limit of the absorption series. If, in any particular 

 case, the frequency of this limit fits the empirical formula 

 for one of the series of terms characteristic of the spectrum 

 of the atom, it becomes very probable that the normal state 

 is of essentially the same character as the other stationary 

 states of the corresponding set. For example, the alkali 



* A. Landed Phys. Zeitschr. xx. p. 233 (1919), and xxi. p. 114 (1920). 



