3i6 



NATURE 



[August 2. 1900 



at the hands of experienced systematists. The con- 

 cluding chapter, dealing with the bearing of palaso- 

 botanical evidence on plant evolution, is full of interest, 

 and particularly valuable as being written by one who 

 possesses both a wide knowledge of the available data and 

 the power of critically weighing the evidence. Referring 

 to the comparative study of species of fossil plants, 

 Zeiller writes : 



" Les Esp^ces, commes les genres, se succMent par 

 vole de substitution et non par voie de transformation 

 graduelle, et il en parait etre de meme ktouslesniveaux." 



A very useful bibliography of writings referred to in 

 the text is given at the end of the volume. A. C. S. 



PHOTOGRAPHY IN NATURAL COLOURS. 

 Lehrbuch der Photochromie {Phoiographie der natiirlichen 

 Farben). Von Wilhelm Zenker ; neu herausgegeben 

 von Prof. Dr. B. Schwalbe. Pp. xiii + 157. (Braun- 

 schweig : Vieweg und Sohn, 1900 ) 

 IN 1868, after long study and repetitions of Edmond 

 Becquerel's experiments on photochromy. Dr. 

 Wilhelm Zenker himself printed and published a " Lehr- 

 buch der Photochromie," which contained a physical 

 explanation of the colour-correctness of these photo- 

 chromatic images. Zenker's book did not have a wide 

 circulation— it would be difificult, perhaps, to name any 

 one in England who has read it— and it was not until 

 J890 that Lippmann, by founding a new method on the 

 principle suggested by Zenker, drew a slightly increased 

 attention to Zenker's labours. That the attention was 

 only slightly increased was due to two causes : firstly, 

 the rather astonishing results of Lippmann and others 

 helped to overshadow the principle of Zenker in the eyes 

 of most people ; secondly, among all those whose pur- 

 suits have any claim to be considered as scientific, 

 English photographers are especially noticeable for their 

 deliberate ignorance of the creative work of the past in 

 photography. 



For the latter re ason, chiefly, the present writer has 

 given, during the past year, a rather full analysis of 

 Zenker's work in " Camera Obscura," and we have now 

 a reprint, in good English type, of the book. In the 

 words of the preface, " The more modern researches on 

 photography in natural colours have shown that the way 

 and the explanations of modern attempts are connected 

 in many respects with Zenker's ideas." The volume 

 contains besides a portrait of Zenker, a sketch of his 

 life and index of his works by Prof. Gustav Krech ; and, 

 finally, Herr E. Tonn gives (pp, 131-157) an account of 

 the further development of photochromy on the founda- 

 tions of Zenker's theory. We shall notice these briefly in 

 their order ; but, with regard to the " Lehrbuch " itself, 

 shall abstain from entering at all fully into the subject 

 of its contents, as in the above-cited reference there is 

 already a full account of it in English. 



Wilhelm Zenker (1829-1899) cultivated many different 

 branches of knowledge. His first papers (1850-1866) 

 were zoological ; the " Lehrbuch " was his first contribu- 

 tion to photography; and his other papers were on 

 colour-perception (1867), photography and physical 

 optics, astrophysics, and, in later life, meteorology. The 

 " Lehrbuch," however, is probably the most important of 

 NO. 1605, VOL. 62] 



his works, and it is to be hoped that now, with this 

 excellent reprint, his methods will have some influence 

 on English photography.^ 



The book is divided' into three parts: (i) Consider- 

 ations on colours ("Das Wesen der Farben"); (2) 

 Account of his predecessors' work in photochromy (" Die 

 Wiedergabe der Farben ") ; and the third part (" Theorie 

 der Photochromie"), after an account of the theories of 

 Seebeck, Becquerel and others, contains Zenker's own 

 ideas (pp. 1 16-129). There are one or two useful notes 

 to this section. 



Herr Tonn's section, with one exception, seems very 

 complete, and full references are given. It is, however, 

 a pity that the very pregnant hint of Lord Rayleigh 

 should be unnoticed {Phil. Mag., 1887). Lord Rayleigh, 

 independently of Zenker, and starting from totally 

 different considerations, indicated in a footnote the 

 Zenker principle, and even went farther ; for not only 

 did he seek to explain ithe results of Becquerel by 

 this principle, but seemed to see the possibility of a 

 new method of photochromy based on it. It would be 

 interesting to have some account of Lord Rayleigh's 

 then promised experimental investigations. If Herr 

 Tonn knew this paper, it is difificult to understand how 

 he resisted the temptation of comparing Rayleigh and 

 Zenker— Zenker who was so clearly a non-mathematician. 



The chief value of the book, the writer persists in 

 believing, is not historical — for it has not had very much 

 influence in the moulding of thought — but is in its spirit ; 

 the influence of its point of view and methods is needed 

 above all at the present time for English photographers ; 

 this does not mean, of course, the small number of 

 English photo-chemists. 



Philip E. B. Jourdain. 



OUR BOOK SHELF. 



Die Harze und die Harzbehdlter. By A. Tschirch. Pp. 



viii 4- 417. (Leipzig : Gebriider Borntraeger, 1900.) 

 The author has spent eight years in collecting and 

 arranging the scattered facts relating to the obscure 

 group of organic compounds which are classified as 

 resins by virtue of a common physical characteristic. 



What Kekule termed "the chemical lumber room" 

 contained at one time a collection of similar obscure 

 groups, such as the alcaloids, colouring matters, tannins, 

 aromatic compounds, &c. ; but since the year when that 

 chemist gave to the world his benzene formula, the 

 lumber room has been industriously ransacked and its 

 contents dragged forth into the light of day. Perhaps 

 the resins have received the scantiest share of attention ; 

 partly, no doubt, owing to the practical difficulties which 

 they offer to the chemist. 



We know nothing of the molecular state which finds 

 its physical expression in these amorphous, translucent 

 compounds, nor how to bring them into a condition of 

 ascertained purity. How often does a promising research 

 miscarry by the unwelcome appearance of resinous 

 products ! Nevertheless the mass of research which has 

 accumulated on the subject fills 400 closely printed pages. 



A great amount of this research gives very little 

 indication of the nature of the resins themselves. The 

 older chemists distilled them and obtained products such 



1 His work for the Paris Academy prize in 1868 stands in close relation 

 to his theory of photochromy (see Fizeau's report, Comfit, rend., Ixvi., 

 Ixvii.). Zenker's memoir was never published, and Otto Wiener (Wied. 

 Ann., 1890, 1895) later and independently followed the same train of 

 thought. {Cf. also Cornu, Poincar^, Potier and Berthelot, Cornet, rend., 

 cxii. ; and Drude, Wied. Ann., xli., xliii.;. 



