542 SCIENCE PROGRESS 



in 1 910 by \V. W. Campbell, as a result of a study of the radial velocities of a 

 large number of stars, that there is a progressive increase in the average linear 

 velocities. This remarkable result has been since confirmed by Boss and others 

 from discussions of the proper motions, and has given rise to several tentative 

 but interesting explanations. Then follows an account of Russell's theory of 

 "giant" and "dwarf" stars, of spectral type M, which supposes that this class 

 can be subdivided into two, not in reality related, the one subclass consisting of 

 very luminous, and the other of feebly luminous stars ; the evidence for and 

 against this theory is discussed. Other remarkable phenomena associated with 

 the spectral type are also brought under consideration. 



The utility of star-counts, and their value as data in the general statistical 

 investigations which aim at determining the density, luminosity, and velocity laws 

 in the idealised stellar system, is explained. Other data which are necessary are 

 derived from the proper motions of stars within different limits of magnitude, and 

 from the parallax determinations. In these investigations it is assumed that 

 there is no absorption of light in space. Although this is almost certainly not 

 true, the amount of absorption must at most be very small, and its magnitude is 

 so uncertain that no useful purpose would have been served by introducing it. 

 The various methods of deducing the three fundamental laws from the observa- 

 tional data are carefully explained. 



In the last chapter of the book is sketched an attempt to found a dynamical 

 theory of the stellar system, the starting-point being the knowledge derived from 

 the study of moving clusters. 



The author has gathered together in this book, and weaved into a connected 

 whole, the results of numerous researches, both by himself and others, which 

 hitherto were only to be found scattered about in various scientific journals ; the 

 results of the most recent researches available up to the time of going to press 

 have been incorporated. Although fresh facts are continually being discovered 

 and our knowledge is increasing rapidly ; and although the new light which will 

 in the course of time be thrown upon many of the points discussed may lead to a 

 recasting of our ideas, it is well to have a logical presentation of the advances of 

 recent years by one who has contributed so much to them, and a survey of the 

 present state of our knowledge. The book will be of great value not only to the 

 student, but also to the original investigator who needs to obtain information upon 

 any particular point ; whilst the valuable list of references and bibliography given 

 at the end of each chapter indicate where fuller information may be obtained by 

 those who desire it. The book, however, should appeal to a wider circle than the 

 student and investigator, for the general scientific reader will find the information 

 presented in a clear and simple manner that will appeal to him, nearly all of the 

 mathematical matter which it was necessary to include having been collected into 

 two chapters, which can be omitted by him with but little detriment to the 

 general argument. 



One misprint may be noted : on p. 158, table 24, " kms. per sec." should head 

 the second column instead of the third. 



H. S. J. 

 PHYSICS 



The Theory of Relativity. By L. Silberstein, Ph.D. [Pp. viii + 295.] 



(Macmillan & Co. Price 10s. net.) 

 The modern theory of relativity was propounded in its essentials by Einstein 

 in 1905, and won immediate recognition on the Continent : nevertheless, the first 

 English book on the subject has only recently appeared. The theory was 



