194 



NATURE 



[July 



though so obvious to an expert, often prove to be a 

 source of infinite trouble to the beginner. 



In dealing with the vexed question of crystallographic 

 notation, we think Dr. Williams has exercised a very- 

 wise discretion. The simple and easily understood 

 symbols of Naumann have been employed in the first 

 instance, but in almost every case the corresponding 

 symbol of Miller's system has been added in brackets. 

 While all students of physics, chemistry, mineralogy, 

 and geology ought to equip themselves with such an 

 amount of crystallographic knowledge as may be derived 

 from the study of this book, only a very small proportion 

 of them are likely to be called upon to deal with the 

 higher and more complicated problems of the science. 

 The small minority of students who devote themselves to 

 purely crystallographic researches may be fairly recom- 

 mended to employ from the first the beautiful method of 

 notation devised by Whewell and perfected by Miller ; 

 but it is more than doubtful if the student with a smaller 

 amount of mathematical training would gain any real 

 benefit from such a course. In an appendix, " on zones, 

 projection, and the construction of crystal figures," the 

 author of this work has indicated to such a beginner the 

 nature of some of the methods of investigation which are 

 pursued by more advanced students. 



In any future edition of the work — and such, we feel 

 sure, will certainly be called for — we think that the 

 author would' do wisely to add a table showing the 

 symbols of the chief forms according to all the different 

 systems of notation commonly employed. The student 

 who turns to the classical memoirs of Des Cloizeaux, 

 Mallard, Bertrand, and others of the French school of 

 crystallography, would thus be enabled to avail himself 

 of much valuable literature, which, owing to the employ- 

 ment of an unfamiliar notation, must otherwise remain a 

 sealed book to him. 



We have spoken regretfully at the outset of this notice 

 of the general neglect of crystallographical studies ; but 

 we are compelled to admit that, for this neglect, crystal- 

 lographers themselves are largely to blame. The con- 

 fusion produced by numerous rival systems of notation is 

 answerable for much of that feeling of despair among 

 those who attempt to make themselves acquainted with 

 the subject. If the time has not yet arrived when a uni- 

 form crystallographic language can be agreed upon, much 

 might be accomplished if the plan adopted by the author 

 of this work of giving in every case the symbols according 

 to two systems were followed. This is already done in 

 the Zeitschrift fur Krystallographie, the Neues Jahrbiich 

 fiir Mineralogie, &c., the Journals of the English and 

 French Mineralogical Societies, and several other well- 

 known periodicals. If a conference of the leading crys- 

 tallographers of Germany, France, and England could be 

 held to decide upon the order in which the axes should 

 be taken in writing symbols and other similar arrange- 

 ments which are purely conventional and arbitrary, we 

 might hope to see much of the confusion removed that 

 has so long been a bar to the progress of this most fasci- 

 nating and important branch of science. 



We feel assured that the simultaneous publication in 



this country and in America of so simple and at the same 



time so accurate a text-book of the subject as the work 



we are now considering will do much towards reviving 



NO. I 131, VOL. 44 I 



and difiusing a taste for the study of crystallography 

 The student who masters the contents of this little 

 book will undoubtedly have much more to learn before 

 he is competent to deal with all the higher problems of 

 crystallographic science ; but, however far his researches 

 may be carried in the future — and this is, perhaps, the 

 very highest praise we can give to the book — he will 

 certainly have little, if anything, to ?^«learn. 



John W. Judd. 



PHOTOGRAPHY IN COLOURS. 

 Photographic des Couleurs par la Methode Interftren- 

 tielle de M. Lippmann. By Alphonse Berget. (Paris : 

 Gauthier-Villars et Fils, 1891.) 



THIS interesting little brochure contains an account of 

 the recent achievements in colour photography 

 which have been made so widely known to the English 

 public through the daily papers. Coming from the pen 

 of an "attachd au Laboratoire des Recherches (Physique) 

 de la Sorbonne," we may take this contribution as an 

 authorized exposition of M. Lippmann's work, and as 

 such it will be found useful by physicists, chemists, and 

 photographers, as well as by the general reader who 

 wishes to know the real state of the case concerning this 

 important departure in photographic methods. In a 

 short historical introduction the author calls attention 

 to the previous photochromatic attempts by Seebeck in 

 1810, by Herschel in 1841, by Edmond Becquerel in 

 1848, by Niepce de St. Victor in 1851 to 1866, and by 

 Poitevin in 1865. It is stated that these and all similar 

 attempts were based upon purely chemical methods, the 

 investigators seeking for some sensitive compound which 

 would give chromatic impressions corresponding to the 

 colours impinging on the film. M. Berget adds the 

 important remark: '''^ a priori, ce probleme est irrdalis- 

 able." 



Chapters ii. to v. are devoted to elementary optical 

 principles. Chapter ii. deals with vibratory movements 

 and their propagation, wave-length and period, and 

 sonorous waves. In the third chapter the phenomenon 

 of interference is described and explained ; in the fourth 

 chapter we have sections on the luminiferous ether, the 

 velocity of light, the decomposition of white light by a 

 prism, and Fresnel's theory of the spectrum colours. 

 The subject of complex colours, as distinguished from 

 the pure colours of the spectrum, is also dealt within 

 this chapter, and is of special importance in connection 

 with the colours of natural objects, to which the author 

 devotes a short section. It is pointed out that the prin- 

 ciple of superposition of vibrations holds good in optics 

 as in acoustics, and that just in the same way that the 

 diaphragm of a phonograph can take up and faithfully 

 transmit the extremely complex system of superimposed 

 aerial vibrations produced by the human voice, so the 

 ether transmits the complex superimposed vibrations 

 emanating from coloured objects. In connection with 

 the history of the undulatory theory, the whole credit is 

 given to Fresnel : " L'honneur de donner la premiere 

 th^orie rationelle de la lumiere, en la consid^rant comme 

 rdsultat d'un inouvement ondulatoire, etait reserve a un 

 savant fran^ais : Fresnel." We should like to have seen 

 Thomas Young receive at least an honourable mention. 



