750 Prof. F. Horton and Miss A. C. D.avies on 



Landers calculations, it is the doublet system of spectrum 

 lines (i.e. the lines of the helium series) which correspond 

 to the coplanar system of orbits, and the single line (or 

 parhelium) series which correspond to the crossed system of 

 orbits. It has been pointed out by Franck and Knipping 

 that the interval between the first radiation potential dif- 

 ference ' (204 volts) and the ionization potential difference 

 (taking for this the value 25*2 volts) is equal to 4'8 volts,, 

 which agrees well with the energy 4*78 volts corresponding 

 to the limit of the helium principal series. Thus this series is 

 associated with the orbit into which the electron is displaced 

 by the absorption of 20*4 volts energy, and, as has been said 

 above, this series corresponds to the coplanar system of 

 orbits. Therefore the abnormal atom resulting from the 

 absorption of 20'4 volts energy is of the coplanar type, while 

 the atom resulting from the absorption of 21 2 volts energy 

 is of the crossed type. The selection principle of Bohr, 

 extended by Sommerfeld and Rubinowitz *, indicates that 

 transitions between orbits of the two systems are not possible,, 

 and the fact that the helium spectrum shows no lines which 

 are combination lines of the helium and parhelium series, 

 affords confirmation of this. 



Lande supposes that in the normal state of the helium 

 atom the two electrons revolve about the nucleus in coplanar 

 orbits, the direction of revolution being the same for both 

 electrons, and the orbit of the outer electron being that 

 corresponding to the term (1, s) of the sharp series. The 

 value of the energy required for ionization of the atom,, 

 calculated from this theory, is however not in agreement 

 with the value determined experimentally, so that Lande's 

 model of the normal helium atom may be rejected for the 

 same reason as Bohr's. More recently Franck and Reiche f 

 have proposed a model of the helium atom in which, the 

 normal orbit of the outer electron is identified with that 

 corresponding to the term (1, Sj of the sharp series of the 

 crossed system. They point out that if this model is the 

 correct one, helium which has once been converted into 

 the coplanar condition cannot, in accordance with the 

 selection principle, revert to the crossed condition, so that 

 the abnormal atoms produced by collisions with electrons 

 having 20"4 volts energy cannot return to the normal 

 condition thereby emitting radiation. They describe the 

 condition of the helium atom in the coplanar state to which 

 it is changed by this absorption of energy as "metastable. ,> 



* See A. Sommerfeld, 'Atombau und Spektrallinien.' 



t J. Franck and 0. Reiche, Zeits. f. Phys. 1. p. 154 (1920). 



