114 



certain cases a transitory stage of development of the inner groups of electrons. 

 Such gradual developments are responsible for the appearance of families of elements 

 like the iron metals and the rare earths. Characteristic examples of the effect of a 

 development of this kind on the structure of spectra are revealed in a recent im- 

 portant paper of Catalan on the series spectra of manganese'). 



As far as the discussion of the individual spectra is concerned, ali-eady in the 

 case of helium the statements must be changed in essential points. Even if we 

 must, indeed, maintain that the peculiar properties of the first order helium spectrum 

 rest in the fact, that the orbit of the inner electron, on account of its approximate- 

 ly simply periodic character, is influenced in a much more thoroughgoing way by 

 the forces originating from the outer electron than is the case in the inner system 

 for other series spectra, the interpretation of the origin of the parhelium spectrum 

 as well as of the normal state of the atom indicated here, is to be fundamentally 

 changed. Concerning the first point Lande ^) has in two papers investigated the 

 helium spectrum, coming to the conclusion that only to orthohelium are to be as- 

 ci-ibed stationary states in which the two electrons move in one and the same 

 plane;, that, however, in the case of parhelium the orbital planes make an angle 

 with each other. Although the discussion of the perturbations given by Lande is 

 open to serious objections, yet the continuation of the work by Dr. Kramers and 

 the writer has led to conclusions which are in general agreement with these chief 

 results. Concerning our detailed calculations, the publication of which has hitherto 

 been put off due to various circumstances, we hope to report in the near future. 



Concerning the normal state of the helium atom, the important experimental 

 researches of Franck and his co-workers^), published in the meantime, have shown 

 conclusively that this state can not be a simple ring configuration as assumed in 

 the text and does not at all belong to the coplanar orthohelium states, but must 

 be regarded as the final state of the binding process corresponding to the emission 

 of the parhelium spectrum. It appears possible to throw light on this point, by 

 means of the correspondence principle, in a way which seems to offer a clue to 

 the understanding of the stability of atomic structure in genei-al. This is thoroughly 

 reported on in Essay III where it is shown that for the discussion of this stability 

 ideas based on ordinary mechanics do not suffice, and that we are led to a view 

 on atomic structure which presents a type of symmetry of the electronic motions 

 essentially different from that of the simple ring configurations previously adopted. 

 From the considerations given there it will also be inferred, that the interpretations 

 given in the text of the lithium spectrum and the spectra of other elements are to 

 be changed essentially. 



Finally concerning the question, touched on at the end of the paragraph, of 



•) Catalan, Trans. Roy. Soc. A 223, p. 127 (1922). 

 2) A. Lande, Phys. Zeitschr. 20, p. 228 (1919) and 21, p. 114 (1920). 



°) J. Franck and F. Reiche, Zeitschr. für Phys. 1, p. 154 (1920), J. Franck and P. Knipping, Zeit- 

 schr. für Phys. 1, p. 320 (1920). 



