164 SCIENCE PROGRESS 



matter, with a brief reference to the recent relativity principle, concludes the 

 volume. 



An excellent feature of the book is the collection of examples at the end of 

 each chapter ; there is none of the artificiality of the average " Examples in 

 Physics" about them. They are akin to the problems which present themselves 

 in a laboratory, and are obviously the invention of one who has been in close touch 

 with practical work. 



We have nothing but the warmest praise for this admirable treatise, and hope 

 that it will speedily find its way into the hands of the students at our Universities. 



J. Rice. 



Relativity and the Electron Theory. By E. Cunningham, M.A. [Pp. v + 96.] 

 Monographs on Physics. (Longmans, Green & Co., 191 5. Price 4s. net.) 



THIS little book deals with a somewhat narrower subject than the author's recent 

 work The Principle of Relativity which was reviewed in last July's issue of 

 SCIENCE PROGRESS. The Relativity principle is a very general hypothesis, viz. 

 that it is impossible by means of physical experiments to determine the absolute 

 velocity of a body through space. The acceptance of this hypothesis involves 

 a very considerable change in our notions concerning the measurement of 

 space and time, and in a certain sense it involves a mutual dependence of those 

 two modes of perception, which have hitherto been regarded as independent 

 of one another. The principle itself is based on the failure of a number of 

 now classical researches to detect and measure the velocity of the earth relative 

 to the ether. Some of these experiments date back to the days of Fresnel 

 and Arago, and naturally such failures were the occasion of much discussion 

 before the advent of Einstein's proposition ten years ago. A fair amount of 

 success had attended various efforts to account for the absence of expected effects, 

 by the invention of hypotheses concerning the constitution of matter. Lorentz and 

 Larmor had rendered signal service in several essays, in which they sought to 

 found a complete theory of matter on the hypothesis that the electron was its 

 fundamental constituent. Einstein, by a flash of insight, reversed the procedure ; 

 instead of inventing ad hoc hypotheses concerning material structure to account 

 for the failure to detect the earth's motion through the ether, he affirmed that 

 these failures were but a sample, as it were, of the failures which would await any 

 attempt to detect absolute motion in space by whatever experimental device the 

 attempt might be made. That postulate, he said, must form the starting point for 

 a revision of our notions of space and time, and thereafter, for a re-discussion of 

 such knowledge, as is summarised, for example, in the equations of the electro- 

 magnetic field. 



As the experimental failures referred to above occupy a very prominent place 

 in the older Electron theory, and in Einstein's postulate, it is not surprising that 

 many of the results of the Electron theory, arrived at as approximations in 

 Lorentz's analysis, appear in the Relativity theory as exact results, and the 

 monograph under review has for its object an explanation of the relation between 

 the two theories and a demonstration where and how the principle of Relativity 

 is "the natural and necessary complement" of the Electron theory. The book 

 accomplishes its purpose in the same able manner as the author's earlier work 

 dealt with the wider aspects of the principle. It is addressed to a wider public 

 than the earlier volume, and more especially to those experimental physicists 

 whose training in mathematical analysis is so insufficient as to render parts of that 

 book unreadable. The origin and development of the principle are sketched from 



