The Principle of Relativity. Chicago: Open Court Publishing Co., 1913. COMSTOCK, D. F. The Principle of Relativity. _Science_, May 20, 1910, vol. 31, p. 767. CUNNINGHAM, E. Einstein’s Relativity Theory of Gravitation. _Nature_, Dec. 4, 11, and 18, 1919. An interesting non-mathematical discussion of the latest phases of the theory. EDDINGTON, A. S. Einstein’s Theory of Space and Time. _Contemporary_ _Review_, Dec, 1919. Good popular article. EDDINGTON, A. S. Gravitation. _Scientific American Supplement_, July 6 and 13, 1918. An excellent popular explanation by the leading British disciple of Einstein. EINSTEIN, A. Time, Space, and Gravitation. _Science_, Garrison, 1920, Jan. 3, n.s., vol. 51, p. 8-10. My Theory. _Living Age._ Boston, 1920, vol. 304, p. 41-3, Jan. 3. FLAMMARION, CAMILLE. Lumen. New York: Dodd, Mead and Co., 1897. Contains nothing about Einstein but presents the relativity of time in fantastic form. KEYSER, C. J. Concerning the Figure and the Dimensions of the Universe of Space. _Science_, June 13, 1913. LODGE, SIR OLIVER. The New Theory of Gravity. _Nineteenth Century_, Dec., 1919. The Ether versus Relativity. _Fortnightly Review_, Jan., 1920. Admirable article by a courteous opponent. POINCARÉ, HENRI. Science and Method; also contained in The Foundations of Science. New York: The Science Press, 1913. RUSSELL, BERTRAND. The Relativity Theory of Gravitation. _English Review_, Dec., 1919. A clear explanation by one of the foremost of British philosophers. THOMSON, J. Deflection of Light by Gravitation and the Einstein Theory of Relativity. _Scientific Monthly_, Garrison, N. Y., 1920, vol. 10, p. 79-85, Jan. VARIOUS WRITERS. The Fourth Dimension Simply Explained. New York: Munn and Co., 1910. The essays submitted for a prize offered by the _Scientific_ _American_. Twenty-two mathematicians try their best to justify the title and if they do not succeed it is not their fault. WETZEL, REINHARD A. The New Relativity in Physics. _Science_, New York, 1913. New ser., vol. 38, pp. 466-474. Explains the relativity of time with diagrams and references to the literature. Other articles of general interest may be found in: _Science_: July 16, 1909; May 20, 1910; June 20, 1913; April 24, 1914; Dec. 5, 1919. _Scientific American Supplement_: April 7, 1917; Dec. 17, 1910. London _Nation_: Nov. 14 and Dec. 27, 1919. London _Times_: Nov. 8, 18, and 25, Dec. 4 and 19, 1919. London _Nature_: Almost every number in Nov., Dec., 1919, Jan., Feb., 1920. New York _Times_: Nov. 7 and 16, Dec. 21, 1919. New York _Sun_: Nov. 10, 1919. _The New Republic_: Jan. 21, 1920. _For the mathematical reader_: EINSTEIN, ALBERT.