PROF. H. A. WILSON is best known for his experimental researches, but this book shows that he is also able to give clear expositions of the more theoretical aspects of modern physics. As he has himself recognised, the title is elastic, and not everyone will agree with his interpretation of it. In particular, most examinations demand a greater knowledge of the newer experimental methods and less of mathematical physics than is given here. The outlines of electromagnetic theory and electron theory are especially good, and furnish an excellent-introduction to more pretentious treatises, whilst the two chapters on relativity are complete in themselves. The sections on the conduction of electricity through gases are good so far as they go, especially the chapter on flames, but too great weight has been given to the work of the Oxford school, and the treatment of the glow-discharge could well have been entirely replaced by an account of the precise methods for studying ionised gases at low pressures that have been developed in the last few years at Schenectady and at Princeton, the potentialities of which have still to be properly recognised.Modern Physics.By Prof. H. A. Wilson. (The Student's Physics, Vol. 6.) Pp. xiv + 381. (London, Glasgow and Bombay: Blackie and Son, Ltd., 1928.) 30s. net.