﻿844 Mr. J. H. Van Vleck on the normal Helium Atom 



model (fig. 2) the two electrons describe an oscillatory 

 motion in a plane, always being symmetrically situated with 

 respect to an axis passing through the nucleus*. In the 

 Lande t and the Franck and Reiche J models the electrons 

 have more complicated orbits o£ unequal size, which are 

 coplanar only in the case of the Lande model (fig. 3). 

 In the Langmuir double-circle model (fig. 4) the orbits are 

 circles lying in two parallel planes *. 



Fig. 1. 



Fig-. 2. 



Fig. 3. 



Fig. 4. 



-e 



-e 



The dimensions of these models must be determined by 

 applying the Sommerfeld- Wilson quantum conditions, which 

 state that 



\pdq=nh, 



where h is Planck's constant, n is an integer, q is an appro- 

 priately chosen generalized coordinate, and p is its conjugate 



momentum —^ . The integration is to be extended over a 



* Physical Review, vol. xvii. p. 339 (Mar. 1921). 



f Phys. Zeits. xx. p. 233 (1919) and xxi. p. 114 (1920). 



\ Zeits. /'. Phys. i. 2, p. 154 (1920) ; Phil. Mag. vol. xlii. p. 125. 



