A. Sommerfeld. Atombau und Spektrallinien, 4th ed. Braunschweig, 1924.

A. Sommerfeld. The Structure of the Atom and Spectra, 4th ed.1

The fourth edition, in comparison with the third, has once again been substantially revised. Unfortunately, it has not been possible to avoid an increase in length, although I tried to exclude everything that could be excluded. Thus I omitted the exposition of crystal structures and molecular models, and shortened the theory of the spherical wave and its momentum in the text and in the appendices; this theory now seems to me unsatisfactory as a basis for quantum radiation. I also struck out the special paragraph on the theory of relativity for the opposite reason, because that theory may be regarded as an acknowledged and universally known foundation of natural science.

It seemed especially important to me to include two subjects in the new edition: Bohr’s theory of the periodic system and the complex structure of spectra. In order that, in the chapter on the periodic system, there might already be a ready conception of a component part of Bohr’s models, the so-called \(n_k\)-orbits, the former fourth chapter on hydro-

native spectrum should have become the second chapter; historically, the \(n_r\)-orbits appeared first of all in Balmer’s theory, and moreover in the simplest form. Unfortunately, already after this part of the book had been written (August 1923), new spectroscopic facts were discovered which have already enriched the exposition of atomic models in the third chapter. For example, from Fowler’s investigation of the spark spectrum of carbon, it can no longer be considered that in the carbon atom there are four equivalent \(2_1\) orbits in the outer shell, as had previously seemed evident on the basis of chemical systematics. Further, in the spectroscopic investigation of titanium and vanadium, the fundamental orbits of these atoms turned out to be of type \(4_4\), although according to Bohr’s scheme only the \(M\)-shell is completed here, while in the \(N\)-shell one could expect, at most, only \(4_1\) orbits. With the deepening of spectroscopic analysis, other contradictions with the original forms of Bohr’s atomic models will undoubtedly be discovered. However, I am convinced that in its general outlines this conception is correct; it takes excellent account of the general chemical and spectroscopic facts. We must frankly admit that the process of binding electrons sketched by Bohr must be subject to all sorts of exceptions according to the energy relations in the outer regions of the atom; but these peculiarities, with the further development of the process, again disappear in accordance with the more typical forms of intra-atomic relations. Exceptions of this kind are to be expected all the more because Bohr’s system is not mathematically grounded, but is intuitive in character.

A major step forward is Stoner’s modification of the Bohr system (Phil. Mag., October 1924); in it, in particular, the difficulty with the atom \(C\) is resolved in an extremely satisfactory manner. According to Stoner, the shells within the atom are further subdivided; the numbers of electrons in the subgroups of atomic shells are different and are determined by formal rules of the internal quantum numbers; the structure of the atom is linked still more closely than before with the structure of X-ray spectra.

Having confined myself essentially only to arranging the facts in accordance with the quantum laws, and omitting atom-mechanical conjectures, I hope to achieve the result that the exposition will not become obsolete too quickly.

As before, the most important and obscure question of the theory remains the nature of light. Formerly I retained the wave theory for purely processes of propagation of light as far as this was possible, but the Compton effect more and more compels me to take the position of an extreme theory of light quanta. I placed the exposition of the Compton effect in the first chapter alongside other fundamental facts of experiment; this is probably the most important discovery that could have been made in the present state of physics.

1. The changes introduced into the 4th edition of this well-known book are best characterized by the author himself. In view of this we give here, in translation, an extract from the preface. — Ed. ↩