Manne Siegbahn. Spektroskopie der Röntgenstrahlen. Pp. VI + 257. Berlin. J. Springer. 1924.

M. Siegbahn. X-ray Spectroscopy.

X-ray spectroscopy is one of the youngest physical disciplines—it has scarcely twelve years of work behind it. Its foundation was laid by two classic papers of the untimely deceased Moseley, whom someone very aptly characterized by Newton’s phrase, spoken on the occasion of Cotes’s death: “If he had lived, we should now know something.” In these works of Moseley not only was the method almost completely outlined and extensive numerical material obtained, but also that relation between frequencies and atomic numbers was found which plays such an outstanding role in atomic physics.

On this firm foundation the edifice of X-ray spectroscopy was erected by the labors of a number of physicists, among whom one should mention

to name M. de Broglie, E. Wagner and, most of all, the author of this monograph. A pupil and successor of Rydberg, Siegbahn inherited from him the traditions of the best optical spectroscopists. In his laboratory—first in Lund, and more recently in Uppsala—the apparatus and methods of precise X-ray spectrometry were most fully developed. He and his collaborators discovered two new series (the M and N series), and developed exact methods for studying absorption spectra. The wavelengths obtained by Siegbahn in most cases have six significant figures; the dispersion of the instruments is such that in second-order spectra lines differing in wavelength by 0.4 X-units are freely resolved (the X-unit = 0.001 angstrom), and it is sometimes possible to observe spectra up to the tenth order. In short, through the work of Siegbahn and his school, X-ray spectroscopy has been raised to the level of optical spectroscopy, whose precision has already become proverbial.

Under these conditions the need for a special monograph devoted to X-ray spectroscopy was unquestionable, and, of course, Siegbahn more than anyone else was called upon to write such a monograph. A characteristic feature of his book—incidentally written in an exceptionally lucid style—is that it is the book of an experimental physicist. The description of instruments and methods of observation (Chapter III) is given with almost exhaustive completeness, so that for a researcher working in this field Siegbahn’s book will, of course, become a standard reference. The survey of emission and absorption spectra is abundantly supplied with numerical tables (Chapters IV and V). In the last chapter we should note the presentation of Lindh’s extremely interesting work, which established the dependence of the position of the absorption edge of certain elements ($S$, $Cl$) on the valence with which they enter into a chemical compound.

Chapter VI, devoted to the systematics and theory of X-ray spectra, is less complete. But these questions are set forth with such fullness in Sommerfeld’s well-known book that the author acts quite rightly in touching only on the most fundamental aspects of the matter. Chapter VII is devoted to the continuous X-ray spectrum and is of profound interest not only to physicists, but also to all who have to deal with an X-ray tube.

Finally, in the last chapter, VIII, other methods (besides X-ray-spectroscopic ones) for determining the internal energy levels of the atom are considered—the photoelectric effect, $\beta$-ray spectra, etc.

The book concludes with a large number of tables and a very complete bibliography extending up to 1923.

E. Shpolsky.