His book is based on a course of lectures which I have delivered to graduate students at Princeton University during the last few years. My aim has been to exhibit the extent to which the fundamental facts of physical science may be coordinated by means of the conception of the electron and the laws of electrodynamics. In developing the subject I have started from the most elementary beginnings, and I have therefore found it necessary to include much matter which is to be found in any ordinary text-book of the theory of electricity and magnetism. It is hoped that the lack of conciseness thereby involved may be more than atoned for by the wider circle to which the book may appeal. The course of lectures at Princeton on which the book is founded proved useful as an introduction to the methods of modern mathematical physics in addition to forming a presentation of the results of recent physical discovery. The broad scope of the subject makes it imperative that a good deal of selection should be exercised as to the nature and treatment of the topics considered. In determining these, consideration has been given to importance, interest, and instructiveness, roughly in the order named. The necessary incompleteness is remedied to some extent by references to scientific papers and to other works. These references are intended to supplement the discussion in the text rather than to exhibit the historical development of the subject. Thus many important papers are not referred to. I have tried, however, to be as accurate as possible in any statements which deal specifically with historical matters.