October 4, 19 17] 



NATURE 



83 



to deny that the addition of pure air- to already 

 existing- sanitary services is most desirable. 



Dr. Brend advocates as a remedy the expan- 

 sion or rarefaction of our large towns and the 

 segreg-ation of our factories (a substantial piece 

 of work for the proposed Ministry of Health !). 

 Perhaps in practice the difficulties of removing 

 smoke from existing towns is less than that of 

 removing our populations to better sites. We 

 have sewag'e systems in all our large towns, and 

 the problem of removing waste products of com- 

 bustion may be no g^reater than that of g-etting 

 Tid of the waste products of the body or of the 

 wash-tub. 



The demand for a Ministry of Health, the per- 

 manent staff of which " must consist almost ex- 

 clusively of medical and scientific men," has 

 much to recommend it, but we are rendered 

 somewhat dubious of the practical results of their 

 lucubrations when we find the author remarking 

 of vaccination : " It is open to argument whether 

 this precaution is still essential purely as a pro- 

 phylactic, though it is of course important dur- 

 ing an epidemic among persons hroughi in con- 

 tact 7vith the disease." We wonder whether Dr. 

 Brend has considered the practical steps requi- 

 •site to vaccinate persons brought in contact with 

 the disease when, for instance, a tramp suffering: 

 from modified small-oox has spread the disease 

 in perhaps half a dozen different towns. Nor are 

 •we clear as to the author's reasons for consider- 

 ing that the diagnosis of this disease has im- 

 proved whilst optxjrtunities of observing it have 

 "become enormously lessened. 



As a whole the volume is well worthy of study. 

 A wide range of subjects affecting public health 

 Is discussed — from the sale of abortefacients to 

 the public health duties of the Treasury. 



OPTICAL THEORIES. 

 Optical Theories, Based on Lectures del 



before the Calcutta University. 

 Mallik. Pp. 181. (Cambridge 



ered 

 By Dr. D. N. 

 At the Univer- 



sity Press, 191 7.) Price 75. 6d. net. 



THE subject of physical optics has undergone 

 a strange vicissitude. Not long ago it 

 ranked as one of the great divisions of mathe- 

 matical science ; now it has become almost wholly 

 absorbed by a sister science. The phenomena of 

 optics, by their variety and ever-increasing 

 practical importance, attract and deserve special- 

 ised study ; but the underlying theory can no 

 longer be studied apart from electricity, and 

 the long succession of theories of the aether 

 in the nineteenth centuiy form a closed 

 chapter in the history of science. There seems 

 little likelihood that the chapter will be reopened. 

 In these circumstances the best approach to the 

 subject may be a matter of doubt, depending a 

 ^reat deal on the temperament of the student. 

 [Those who are historically minded will urge that 

 16 present position is best apprehended by fol- 

 NO. 2501, VOL. 100] 



lowing the steps which have led to it; others 

 will consider that adherence to the traditional 

 mode of approach tends to root in the mind an 

 obsolete mode of thoup'ht, and it is better not to 

 trifle with the freedom which is now offered. Dr. 

 M&llik's book offers a compromise which should 

 be acceptable to both sides. A survey of the 

 more essential properties and differences of the 

 mechanical aethers that were once proposed, 

 subordinated to the purpose of illustrating the 

 difficulties which the electromagnetic theory 

 strides over so easily that they pass almost un- 

 noticed ; afterwards, an account of the modern 

 theory and results, contrasted where necessary 

 with the mechanical aethers — the:^e seem to bring 

 out the essential aspects of our present know- 

 ledge, without undue neglect of the lessons of 

 the past. 



Dr. Mallik divides his subject into four prin- 

 cipal chapters : early theories, elastic solid theory, 

 electromagnetic theory, and electron theory. The 

 separation of the two last has some disadvantages. 

 So far as it relates to free aether, the electro- 

 magnetic theory is independent of electrons ; but 

 in the early extensions to dielectric media the 

 object presumably was to evade, rather than to 

 theorise on, the relations of matter to sether. 

 To make a full discussion of the mechanical 

 implications of this approximate treatment seems 

 unnecessary at the present day ; the undeveloped 

 nations of permeability and specific inductive 

 capacity scarcely need to be taken so seriously. An 

 excellent summary of the whole argument is 

 given in the concluding- chapter, which shows 

 how far we have travelled since speculations on 

 the optical medium first began. The theory of 

 relativity and the quantum theory are not included 

 in the scope of the book. 



The volume is written for fairly advanced 

 students, and the discussion necessarily is 

 mainly mathematical. A great amount of work is 

 surveyed in brief compass ; and most readers 

 will find fresh information, and arguments that 

 are new to them. According- to the author's 

 plan, only those developments are treated which 

 afford a means of discriminating between rival 

 theories; and the student will do well to follow 

 his guidance through the bewildering mass of 

 investigations which still confront the learner, 

 as well as through the debris of wrecked hypo- 

 theses. 



OUR BOOKSHELF. 

 Chile. Pp. 301. (Santiago: The Chilean 



Government, 1915.) 

 This anonymous volume written in English is 

 doubtless meant to diffuse a knowledge of Chile 

 and particularly of Chilean resources and trade in 

 English-speaking countries, but there is no pre- 

 ! face to indicate its aim or the personality of its 

 editor. In about fifty short chapters it contains 

 a comprehensive survey of Chile, its life and 

 conditions, including a good deal of statistical 



