EXCLUSION PRINCIPLE AND QUANTUM MECHANICS 29 was of decisive importance for the finding of the exclusion principle Very soon after my return to the University of Hamburg in 1923, I gave there my inaugural lecture as Privatdozent on the Periodic System of El ements. The contents of this lecture appeared very unsatisfactory to me since the problem of the closing of the electronic shells had been clarified no further. The only thing that was clear was that a closer relation of this prob- lem to the theory of multiplet structure must exist. I therefore tried to exam- ine again critically the simplest case, the doublet structure of the alkali spec- tra. According to the point of view then orthodox, which was also taken over by Bohr in his already mentioned lectures in Gottingen, a non-vanish ing angular momentum of the atomic core was supposed to be the cause of this doublet structure In the autumn of 1924 I published some arguments against this point of view, which I definitely rejected as incorrect and proposed instead of it the assumption of a new quantum theoretic property of the electron, which I called a u two-valuedness not describable classically > At this time a paper of the English physicist, Stoner, appeared which contained, besides improve- ments in the classification of electrons in subgroups, the following essential remark: For a given value of the principal quantum number is the number of energy levels of a single electron in the alkali metal spectra in an external magnetic field the same as the number of electrons in the closed shell of the rare gases which corresponds to this al quantum number On the basis of my earlier results on the classification of spectral terms in a strong magnetic field the general formulation of the exclusion principle be- came clear to me. The fundamental idea can be stated in the following way: The complicated numbers of electrons in closed subgroups are reduced to the simple number one if the division of the groups by giving the values of the four quantum numbers of an electron is carried so far that every degen- eracy is removed. An entirely non-degenerate energy level is already u closed D, if it is occupied by a single electron; states in contradiction with this postula have to be excluded. The exposition of this general formulation of the ex clusion principle was made in Hamburg in the spring of 1925after I was able to verify some additional conclusions concerning the anomalous Zee- man effect of more complicated atoms during a visit to Tubingen with the help of the spectroscopic material assembled there With the exception of experts on the classification of spectral terms, the physicists found it difficult to understand the exclusion principle, since no meaning in terms of a model was given to the fourth degree of freedom ofEXCLUSION PRINCIPLE AND QUANTUM MECHANIC S 29 was of decisive importance for the finding of the exclusion principle. Very soon after my return to the University of Hamburg, in 1923, I gave there my inaugural lecture as Privatdozent on the Periodic System of El￾ements. The contents of this lecture appeared very unsatisfactory to me, since the problem of the closing of the electronic shells had been clarified no further. The only thing that was clear was that a closer relation of this prob￾lem to the theory of multiplet structure must exist. I therefore tried to exam￾ine again critically the simplest case, the doublet structure of the alkali spec￾tra. According to the point of view then orthodox, which was also taken over by Bohr in his already mentioned lectures in Göttingen, a non-vanish￾ing angular momentum of the atomic core was supposed to be the cause of this doublet structure. In the autumn of 1924 I published some arguments against this point of view, which I definitely rejected as incorrect and proposed instead of it the assumption of a new quantum theoretic property of the electron, which I called a « two-valuedness not describable classically » 3 . At this time a paper of the English physicist, Stoner, appeared4 which contained, besides improve￾ments in the classification of electrons in subgroups, the following essential remark: For a given value of the principal quantum number is the number of energy levels of a single electron in the alkali metal spectra in an external magnetic field the same as the number of electrons in the closed shell of the rare gases which corresponds to this principal quantum number. On the basis of my earlier results on the classification of spectral terms in a strong magnetic field the general formulation of the exclusion principle be￾came clear to me. The fundamental idea can be stated in the following way: The complicated numbers of electrons in closed subgroups are reduced to the simple number one if the division of the groups by giving the values of the four quantum numbers of an electron is carried so far that every degen￾eracy is removed. An entirely non-degenerate energy level is already « closed », if it is occupied by a single electron; states in contradiction with this postulate have to be excluded. The exposition of this general formulation of the ex￾clusion principle was made in Hamburg in the spring of 19255, after I was able to verify some additional conclusions concerning the anomalous Zee￾man effect of more complicated atoms during a visit to Tübingen with the help of the spectroscopic material assembled there. With the exception of experts on the classification of spectral terms, the physicists found it difficult to understand the exclusion principle, since no meaning in terms of a model was given to the fourth degree of freedom of