The preparation of this new edition has made it possible to correct
errors, to further amplify certain portions of the text and to
enlarge the ever-increasing bibliography on the subject. Photographs
of Professors J. J. Thomson, Michelson, Minkowski and Lorentz are
also new features in this edition.
The explanatory notes and articles in the Appendix will, I believe,
present no difficulties to readers who have mastered the contents of
the book. They are in fact "popular expositions" of various phases
of the Einstein theory; but experience has shown that even "popular
expositions" of the theory need further "popular introductions."
I wish to take this opportunity of thanking Prof. Einstein,
Prof. A. A. Michelson of the University of Chicago, Prof. J. S. Ames
of Johns Hopkins University, and Professor G. B. Pegram of Columbia
University for help in various ways which they were good enough to
extend to me. Prof. J. S. Ames and the editor of Science have been
kind enough to allow me to reprint the former's excellent presidential
address on Einstein's theory, delivered before the members of the
American Physical Society.