couple of years his theory was claiming the attention of a number of prominent mathematicians and physicists. Minkowski, a German mathematician of the first rank, just at this time turning his attention to mathematical physics, came out in 1909 with his famous world postulate, which has been briefly described. It is interesting to note that within a year translations of Minkowski's article appeared in English, French and Italian, and that extensions of his theories have occupied the attention of a number of Germany's most famous mathematicians. Next Poincaré, perhaps the most brilliant mathematician of the last quarter century, stamped the relativity theory with the unofficial approval of French science, and Lorentz, of Holland, one of the most famous in a land of famous physicists, aided materially to the development of the subject. Thus we find within five years of the appearance of Einstein's article, a fairly consistent body of doctrine developed, and accepted to a surprising degree by many of the prominent mathematical physicists of the foremost scientific nations. No sooner was the theory in a fairly satisfactory condition, than the attempt was made to verify some of the hypotheses by direct experiment. Naturally the difficulties in the way of such experimental verification were very great—insurmountable in fact for many experiments, since no two observers could move relative to each other with a velocity approaching that of light. But the change in mass of a moving electron could be measured, and a qualitative experiment by Kaufmann, and a quantitative one by Bucherer gave results which were in good agreement with the theoretical equations. It was the hope of the astronomers that the new theory would account for the long-outstanding disagreement between the calculated and the observed motion of Mercury's perihelion, but while the relativity mechanics gave a correction in the right direction, it was not sufficient. To bring this very brief historical sketch down to the present time, it will perhaps be sufficient to state that this theory is at present claiming the attention of a large number of prominent mathematicians and physicists. The details are being worked out, the postulates are being subjected to careful mathematical investigation, and every opportunity is being taken to substantiate experimentally those portions of the theory which admit of experimental verification. Practically all of the work which has been done is scattered through research journals in some six languages, bo that it is not very accessible. Some idea of the number of articles published may be obtained from the fact that a certain incomplete bibliography contains the names of some fifty-odd articles, all devoted to some phase of this subject—varying all the way from the soundest mathematical treatment, at the one end of the scale, to the most absurd philosophical discussion at the other. And these fifty or more articles include only those in three languages, only those which an ordinary
