October 31, 1912] 



NATURE 



247 



few special features which demand attention. In 

 the present case httle can be said but that the 

 authors have presented what appears to be a 

 straig-htforward and clear account of the elements 

 of the subject. 



The general method of treatment is based upon 

 the two previous text-books of Dr. Jude, more 

 stress being now laid, however, on the practical 

 side of the subject. Descriptions of a large 

 number of experiments with simple apparatus are 

 inserted in the text, and the diagrams are rather 

 more frequent than is usually the case. It is 

 rather a pity that some of the latter, which are 

 evidently new, have not been printed more clearly. 

 An unsatisfactory feature (which, however, it 

 should in fairness be stated, is by no means peculiar 

 to this case) is the mode of definition of the units 

 in electromagnetism. Even if it be admitted that 

 exact definitions are beyond the scope of this work, 

 a voU should not be defined as " 10/ 1 1 of the electro- 

 motive force of a Daniell's cell" without further 

 explanation. Surely, also, the unit of current 

 could be explained from first principles and not 

 be defined in terms of the volt and the ohm. 



(2) Here, again, we have a further addition to 

 the numerous works on elementary practical 

 physics. This book is, however, rather more 

 advanced than those which have lately appeared, 

 and, although no previous knowledge of physics 

 is assumed, the authors claim that it contains 

 more than sufficient matter for the intermediate 

 examination for degrees, almost enough, in fact, 

 for the pass final examinations. One serious 

 objection to the publication of books of instruction 

 in practical work is that in different laboratories 

 different types of apparatus are usually found, a 

 fact which would tend to limit the usefulness of 

 the book to the students under the direction of the 

 author, who generally bases his work upon his 

 own course. This the present authors have 

 endeavoured to avoid by describing experiments 

 involving the use of standard apparatus, or that 

 which can easily be made. 



The arrangement of the book is excellent. Some 

 120 experiments are described and each exercise 

 is divided as follows : — apparatus, object of experi- 

 ment, theory and results. It is not intended that 

 the instructions printed should form the whole 

 information given to the student. This also is 

 a good feature, for it is certain that no thorough 

 teaching of practical physics is possible if the 

 services of the demonstrator are dispensed with. 



(3) The collection into one volume of the mathe- 

 matical and physical researches of the late M. 

 Potier is certain to direct the attention of men of 

 science, whether physicists, mathematicians or 

 engineers, to the wonderful versatility of the author 



NO. 2244, VOL. 90] 



and to demand their admiration. When it is 

 remembered that M. Potier's geological researches 

 (which are not included in the present collection) 

 have rendered him eminent in that subject also, his 

 wonderful ability becomes still more apparent. As 

 M. Henri Poincare (himself, unfortunately, deceased 

 since the publication of this volume) points out 

 in the preface, Potier was a mathematician, and 

 undertook very little experimental work, but he 

 nevertheless kept in touch with the results obtained 

 by others and endeavoured to conform his mathe- 

 matical researches to the experimental require- 

 ments of the case. Especially is this so in his 

 most important papers on applied electricity, which 

 are compiled in the second part of this volume. 

 Indeed, his services to electrical engineering are 

 regarded as forming the most brilliant part of his 

 work. In the first part the memoirs on electrical 

 theory are collected, and the third part contains 

 those dealing with many and varied questions in 

 light. 



(4) It is indeed surprising (as the translator 

 points out) that no English fedition of this treatise 

 of Huygens has until now appeared. Two 

 hundred and twenty years have elapsed since the 

 original saw light, and in the interval it has pro- 

 duced a more pronounced effect on the science of 

 optics than almost any other work. It is, there- 

 fore, a matter for congratulation that the long- 

 delayed edition is an excellent one, and it will 

 be welcomed by all those who find delight in read- 

 ing the writings of the masters in science. Not 

 the least pleasing feature of Prof. Thompson's 

 translation is the preservation of the old-time mode 

 of statement, which recalls the earlier English 

 editions of Newton's "Principia." Prof. Thomp- 

 son's reason for a literal rendering is, however, 

 more fundamental than that of making reading 

 pleasant. He wished to avoid "importing into 

 the author's text ideas of a subsequent date, by 

 using words that have come to imply modern con- 

 ceptions." The publishers have carried out the 

 idea of antiquity in the type and binding and have 

 produced a really beautiful book. 



(5) Here is a book which will undoubtedly fill 

 a gap in the various publications on physics. 

 There is a large class of students who do not 

 carry their studies in physics beyond the inter- 

 mediate stage, branching off into engineering or 

 medicine at that point. As a general rule, no suit- 

 able text-book provision has yet been possible in 

 such cases. The single volumes on physics already 

 in existence are either too simple or too difficult, 

 and the students in question have objections to pur- 

 chasing separate volumes in the various branches 

 of the subject. It appears fairly certain, there- 

 fore, that this book of Dr. Watson's will play a 



