This book has grown from lectures on X-rays which I have given at the University of California and the University of Chicago during the last five years. Though I have tried to cover the whole field of the physics of X-rays with some completeness, those aspects have naturally been treated in greater detail which are most closely allied with my own researches. Such emphasis is perhaps the more justified by the recent appearance of a new edition of the Braggs' notable book X-rays and Crystal Structure, together with treatises by Wyckoff, Rinne, Ewald and others covering about the same field, and Siegbahn's excellent account of The Spectroscopy of X-rays. In the present volume only an introduction to the problems of crystal structure and X-ray spectroscopy has been given. Kaye and de Broglie in their books on X-rays have described in some detail the experimental aspects of the subject. It is rather with the interpretation of the properties of X-rays in terms of the interaction between radiation and electrons that the present work deals. 1 have been chiefly concerned with the information X-ray studies have afforded regarding the structure of the atom and the nature of the X-rays themselves.