REVIEWS 493 



Some doubt may be expressed as to whether it was wise to omit numerical 

 applications altogether in a subject whose ultimate aim is always numerical 

 expression. Not to have done so would have enormously increased the size of the 

 book. For this reason, therefore, the author is justified in taking a via tnedta. 

 The mathematical formulae have, wherever possible, been reduced to a form 

 suitable for numerical application, and, in doing so, the author's skill as a 

 computer has proved itself of value. There is truth in the introductory remarks : 

 "The student who feels the need will have no difficulty in finding forms of 

 computation in other works. At the same time the reader who will take the 

 trouble to work out such forms for himself will be rewarded with a much truer 

 mastery of the subject, though he should not disdain what is to be learnt from the 

 tradition of practical computers." 



All readers may not agree with the criticisms which we have offered, and much 

 justification can be advanced for the course which Prof. Plummer has adopted. 

 The book is written with a freshness of treatment and a careful elucidation of 

 difficulties which will commend themselves to the student. It meets a long-felt 

 want. The elaborate treatises of Laplace, Tisseraud, Poincare, and other writers 

 on dynamical astronomy are too detailed to enable a student to study them with 

 advantage without previous preparation. Prof. Plummer's book will make the 

 way easier for him by giving him a general knowledge of the whole subject, which 

 will serve as a stepping-stone to the classical treatises. We are grateful to Prof. 

 Plummer for supplying this want in so admirable a manner. 



H. S. J. 



PHYSICS 



Report on the Relativity Theory of Gravitation. By Prof. A. S. Eddington, 

 M.A., F.R.S. [Pp. vii + 91.] (The Physical Society of London, 1918. 

 Price 6s.) 



The first chapter of this Report, dedicated 10 a brief survey of "the older 

 relativity," is somewhat unsatisfactory, lacking, that is, in clearness of presentation 

 and conceptually incorrect. Thus, for example, any reader not already informed 

 will arrive at the end of page 3 with entirely confused or even wrong ideas about 

 the very fundamental concept of the longitudinal contraction. "The universal 

 nature" of this change — says the author — "makes it impossible to perceive any 

 change at all." The reader will thus believe that there is no real content about 

 that change, and that it concerns only the formal, mathematical side of pre- 

 sentation. Now, for the observer sharing the motion of a body (more correctly, 

 with respect to whom the body rests) there is no "contraction" at all, real or 

 formal, and thus there is no question of perceiving it ; but for an outsider there 

 is a real phenomenal change, revealing itself, for example, in the observable 

 properties of electrons (cathode rays), etc. The author fails to make it clear. 

 The following page 4 rather intensifies this obscurity. There are also other 

 minor yet disturbing defects in Chap. I. which cannot be entered upon in a short 

 review. Part of this obscurity pervades, by necessity, the first two or three sections 

 of Chap. II., entitled "The Relations of Space, Time, and Force." Yet it is here 

 of less consequence. At this stage (pp. 18-19) the view taken of a "straight 

 line" is rather objectionable, although not seriously hindering the progress of the 

 Report. Very defective, however, is the enunciation (wording) of Einstein's 

 equivalence-hypothesis. Gravitation is not so much something " introduced by 

 the transformation of the co-ordinates of reference " as by the changed metrical 



