THE second edition of Vol. 2 of Prof. Haas's “Introduction to Theoretical Physics” deserves special mention, because in the four years which have elapsed since the publication of the first English edition (NATUBE, vol. 117, p. 687) great strides have been made in the development of atomic theory. The changes in outlook consequent on the introduction of the wave theory of matter and of the new quantum mechanics are reflected in Part III., which has been almost entirely rewritten. It is significant of the new point of view that the chapter on the principles of atomic mechanics begins with a description of de Broglie's theory, which is followed by an account of Schrodinger's theory. It is not until the following chapter that we meet with a detailed statement as to atomic spectra, and the explanation of the spectrum of hydrogen by means of Bohr's model. Prof. Haas has written a separate volume on “Wave Mechanics and the New Quantum Theory”, in which these subjects are treated at greater length, but it is convenient to have such a concise and clear account of the new developments as that given 4 in the book here noticed. Introduction to Theoretical Physics. By Prof. Arthur Haas. Translated by Dr. T. Verschoyle. Vol.2. Second edition. Pp. xi + 492. (London: Constable and Co., Ltd., 1929.) 21s. net.