WRITERS on astronomical subjects find it comparatively easy to give a popular or semi-popular description of such a parochial affair as the solar system, but not so easy to provide a similar description of the Galaxy. The book under notice is undoubtedly the best of the semi-popular type that has been produced on this subject, and it presents the reader with an excellent account of the methods used to explore the Milky Way and to fathom some of its secrets. Here we have descriptions of the means by which astronomers ascertain stellar distances, masses, sizes, temperatures, etc., and most of these can be easily understood by those who have little or no background in mathematics or physics. They will read the interesting story of our Galaxy-just one of millions-the longest axis of which is more than 100 million light-years, the number of stars probably exceeding 100,000 million. Some of these stars are so large that the sun could be placed at the centre, and the orbit of Mars would lie well inside their outer layers, while the whole system is moving around its centre, the speed of the sun being 150 miles a second and the time required to complete a revolution 200 million years. The Milky Way By Bart J. Bok Priscilla F. Bok. (Harvard Books on Astronomy.) Second edition. Pp. vi + 224. (London: J. and A. Churchill, Ltd., 1946.) 18s.