IN a subject that has been developing as rapidly as that of cosmic rays, the writing of books is a difficult and often not very fruitful task. Anything in the nature of a treatise is as yet impossible, and treatment of special branches may be out of date by the time a book can be prepared and published. But special circumstances have enabled the author of this work to avoid this pitfall. The small size of the Cambridge Physical Tracts, in this case only 65 pages, calls for extreme condensation, but makes possible rapid publication. Revision has been made to June 1939, so the book may be considered as up to date as is humanly possible. The author does not pretend to cover completely the whole subject. The first part is a useful digest of the more significant experimental results, while the second is a review, from the theoretical side, of the passage of cosmic rays through matter, and in particular of the shower phenomenon. One chapter is devoted to the mesotron, and contains the evidence for assuming the existence of a new particle of mass intermediate between the proton and the electron. There is also a short discussion of its theoretical implications.
Cosmic Rays and Mesotrons
By Dr. H. J. J. Braddick. (Cambridge Physical Tracts.) Pp. x + 68. (Cambridge: At the University Press, 1939.) 5s. net.