August i8, 1923] 



NATURE 



233 



of the theory, in their preoccupation with the extension 

 of the subject, have not had too much pity on their 

 weaker brethren, and it has not always been easy to 

 discover either the exact nature or the physical basis 

 of some of the principles to which they appeal. M. 

 Brillouin's lucid and authoritative survey of the 

 whole subject is, therefore, particularly welcome and 

 valuable. 



It was a happy inspiration on the part of M. Brillouin 

 to preface his main thesis with two preliminary chapters 

 on the quantum theory of radiation. It is a subject on 

 which the author has himself done much valuable work, 

 and his excellent, though brief, account provides a 

 firm basis for the developments which follow. The 

 succeeding chapters on the theory of Bohr, on its 

 applications to atomic structure, and in particular the 

 account of the principles of selection and correspond- 

 ence, are equally illuminating. 



It is not to be expected, from the very nature of the 

 subject, that the volume should be easy reading. The 

 author has not shirked the very considerable mathe- 

 matical difficulties which are involved in the theory. 

 He has, however, minimised them as far as is consistent 

 with a proper understanding of the argument. Though 

 it cannot be promised that the average student of 

 physics will find his progress through the volume an 

 easy one, he may be assured that his labours will be 

 rewarded by a completer knowledge and a deeper 

 appreciation of this important subject. 



(3) It is in no way derogatory to M. Brillouin's 

 excellent treatise to say that it is surpassed in interest 

 by the report of the proceedings of the council of dis- 

 tinguished physicists who assembled in Brussels in 192 1 

 under the presidency of Prof. Lorentz and under the 

 auspices of the Solvay Institute. The number and 

 distinction of the participants, each a master in his 

 own particular branch, and the variety and importance 

 of the subjects considered would in themselves suffice 

 to raise high expectations. It may be said at once 

 that, in the main, these expectations are fully realised 

 by the volume which is now to hand. 



Each of the twelve closely related subjects chosen for 

 discussion was introduced at the Conference by a report 

 on the actual position of the subject, and these reports 

 make up the main part of the text. Thus Sir Ernest 

 Rutherford reports on the structure of the atom, 

 M. de Broglie on the quantum relation in the photo- 

 electric effect, Prof. Kamerlingh Onnes contributes an 

 account of his work on paramagnetism at low tempera- 

 tures and on the super-conductivity of certain metals 

 at low temperatures. Prof. Bohr gives an account of 

 the application of the theory of quanta to atomic 

 problems, which is supplemented by a report from 

 Prof. Ehrenfest on the principle of correspondence. 

 NO. 2807, VOL. I 12] 



It is natural that the different authors should develop 

 their subjects in slightly different ways, and should 

 assume slightly different degrees of previous knowledge 

 amongst their distinguished colleagues, or perhaps we 

 should rather say among the wider circle of readers for 

 whom the reports were ultimately destined. In most 

 cases, however, the reports are so well conceived and so 

 lucidly expressed that the reader with only an element- 

 ary knowledge of the subject will have little difficulty 

 in following a very considerable part of them. It is, 

 in fact, an open question whether such a reader, at 

 the expense of a httle judicious " skipping " of the 

 more recondite portions, would not attain a better 

 appreciation of the present position of atomic physics 

 from this volume than from many of the works osten- 

 sibly written for his special benefit. This, of course, 

 does not apply to the one or two reports of a mathe- 

 matical character, such as the profound suggestions of 

 the president, Prof. Lorentz, in his notes on the theory 

 of electrons which opens the volume. 



In addition to their expository value, these reports 

 have the great merit of opening up new avenues 

 for discussion and experiment. In dwelling on the 

 very considerable achievements which have been 

 brought about, partly by the appHcation of quantum 

 theories to atomic problems, i t is apt to be overlooked 

 that these theories present formidable difficulties in 

 addition to the fundamental one of explaining them- 

 selves. These difficulties are clearly raised in the 

 discussions which follow the reports, and perhaps in 

 none of them more clearly than in Prof. Barkla's dis- 

 cussion of M. de Broglie's report on the photoelectric 

 effect. The discussions, which are excellently reported, 

 are full not only of scientific but also of human interest. 

 To the physicist, whether mathematical or ex- 

 perimental, in need of a subject for research they 

 offer an ample choice of problems of fundamental 

 importance. 



It is to be regretted that so long an interval has been 

 allowed to elapse between the meetings of the council 

 and the publication of its report. It was not to be 

 expected that the members of the council would allow 

 two years to elapse before attempting the solution of 

 some of the problems raised, and still less to be expected 

 that their attempts should be entirely without success. 

 In some particulars, therefore, the subject has advanced 

 beyond the stage indicated in the reports. In the 

 main, however, this applies only to minor problems. 

 If it is true that intellectual satisfaction results from 

 the discovery rather than from the knowledge of truth, 

 a perusal of this volume will convince the reader that in 

 this portion of physics he may confidently expect to 

 find intellectual satisfaction for many years to come. 



J. A. C. 

 G I 



