Of the non-mathematical articles which have appeared, those by Prof. A. S. Eddington (Nature, volume 101, pages 15 and 34, 1918) and Prof. M. R. Cohen (The New Republic, Jan. 21, 1920) are the best which have come to the author's notice. Other articles on Einstein's theory, some easily comprehensible, others somewhat confusing, and still others full of noise and rather empty, are by H. A. Lorentz, The New York Times, Dec. 21, 1919 (since reprinted in book form by Brentano's, New York, 1920); J. Q. Stewart, Scientific American, Jan. 3, 1920; E. Cunningham, Nature, volume 104, pages 354 and 374, 1919; F. H. Loring, Chemical News, volume 112, pages 226, 236, 248, and 260, 1915; E. B. Wilson, Scientific Monthly, volume 10, page 217, 1920; J. S. Ames, Science, volume 51, page 253, 1920 [12]; L. A. Bauer, Science, volume 51, page 301 (1920), and volume 51, page 581 (1920); Sir Oliver Lodge, Scientific Monthly, volume 10, page 378, 1920; E. E. Slosson, Independent, Nov. 29, Dec. 13, Dec. 20, Dec. 27, 1919 (since collected and published in book form by Harcourt, Brace and Howe); Isabel M. Lewis, Electrical Experimenter, Jan., 1920; A. J. Lotka, Harper's Magazine, March, 1920, page 477; and R. D. Carmichael, New York Times, March 28, 1920. Einstein himself is responsible for a brief article in English which first appeared in the London Times, and was later reprinted in Science, volume 51, page 8, 1920 (see the Appendix). A number of books deal with the subject, and all of them are more or less mathematical. However, in every one of these volumes certain chapters, or portions of chapters, may be read with profit even by the non-mathematical reader. Some of these books are: Erwin Freundlich, The Foundations of Einstein's Theory of Gravitation (University Press, Cambridge, 1920). (A very complete list of references--up to Feb., 1920--is also given); A. S. Eddington, Report on the Relativity Theory of Gravitation for the Physical Society of London (Fleetway Press, Ltd., London, 1920); R. C. Tolman, Theory of the Relativity of Motion (University of California Press, 1917); E. Cunningham, Relativity and the Electron Theory (Longmans, Green and Co., 1915); R. D. Carmichael, The Theory of Relativity (John Wiley and Sons, 1913); L. Silberstein, The Theory of Relativity (Macmillan, 1914); and E. Cunningham, The Principle of Relativity (University Press, Cambridge, England, 1914). To those familiar with the German language Einstein's book, Über die spezielle und die allgemeine Relativitätstheorie (Friedr. Vieweg und Sohn, Braunschweig, 1920), may be recommended. [13] The mathematical student may be referred to a volume incorporating the more important papers of Einstein, Minkowski and Lorentz: Das Relativitätsprinzip, (B. G. Teubner, Berlin, 1913). Einstein's papers have appeared in the Annalen der Physik, Leipzig, volume 17, page 132, 1905, volume 49, page 760, 1916, and volume 55, page 241, 1918. APPENDIX Note 1 (page 21)