1. This article represents substantially the contents of an address delivered, on August 30, before the sixth Scandinavian Mathematical Congress in Copenhagen. It should be mentioned, however, that the present elaboration of the text has been notably influenced by the appearance, since the delivery of the address, of an important paper by Heisenberg, to which reference is made below.
2. For a closer discussion of these problems reference may be made to a recent article of the writer; compare in particular the addendum (Zs. für Phys. 34, 142, 1925).
3. Compare an article of the writer (Ann. d. Phys. 71, 228, 1923) which contains a general survey of the results regarding the explanation of spectral evidence on the basis of mechanical pictures of stationary states. In this article detailed reference to the earlier literature on the subject may be found, and we therefore have confined ourselves here to quoting papers which have appeared since then.
4. O. Klein, Zs. für Phys. 22, 109, 1924.
5. W. Lenz, Zs. für Phys. 24, 197, 1924.