104 BELL SYSTEM TECHNICAL JOURNAL 



the joining of the first electron to a nucleus of charge -\-2e, the neutral- 

 helium spectrum the adhesion of the second electron to such a nucleus. 

 Spectra corresponding to the latest four or five stages, in the forma- 

 tion of atoms ha\ing many electrons when completed, have been 

 observed. To a certain extent, but not entirely, an atom with Z 

 electrons and a nuclear charge Z resembles an atom with Z electrons 

 and a nuclear charge Z+1. To a certain extent, therefore, each atom 

 in the Periodic System may be regarded as resembling the last stage 

 but one in the formation of the next following atom. This fact is 

 important in the interpretations of the Periodic Table. 



(e) Midtiplets. We next take account of the fact that the sequences 

 of Stationary States, mentioned in the elementary theory and descrip- 

 tion of spectra, are actually sequences of groups of Stationary States; 

 and inquire what may be supposed to differentiate the several states 

 of a group from one another. An elaborate formal theory is based 

 on the assumption that all of the electrons of what I have called the 

 "residue" of the atom revolve, if not literally as a rigid block, at least 

 with a resultant angular momentum which itself is quantized; and 

 that the outermost electron revolves in its own orbit around this 

 residue, the different Stationary States of the group differing from 

 one another in respect of the inclination of the orbit to the axis of 

 rotation of the residue. The theory is not quite coherent with what 

 has gone before; and for that reason the reader should try to separate 

 its essential qualities from its accidental ones. 



(f) Magnetic Properties of Atoms. A magnetic field should treat 

 a system of electrons revolving around a nucleus in the same w^ay as 

 it treats one electron, as was said in the Second Part of this article. 

 One would expect that in this case, if in any, the behavior of complex 

 atoms would resemble that of the hydrogen atom; yet there is a 

 striking and inexplicable contrast. This, like the spectrum of the 

 helium atom, shows that either the quantum conditions governing the 

 hydrogen atom are not universal, or the expressions hitherto found 

 for the quantum conditions are too limited. From the responses of 

 atoms to magnetic fields something is learned about the magnetic 

 properties of atoms and their residues, some part of w^hich can be 

 tested by direct experiment; and these experiments include what are 

 probably, all things considered, the most perplexing and fascinating 

 ones of recent years. 



(g) Interpretation of X-ray Spectra. X-ray spectra are anah'zed 

 as other spectra are, and each absorption and each emission of an 

 X-ray by an atom is associated with a transition between two Station- 

 ary States; these "X-ray Stationary States" however are distin- 



