446 



SCIENCE. 



[N. S. Vol. XII. No. 299. 



resulting from the interference of the direct 

 wave train with the train reflected from the 

 back surface of the film. In other words, the 

 colors of the photochromes were similar to the 

 colors of the soap-bubble. This is precisely the 

 principle since made use of by Lippman in his 

 beautiful process. 



Zenker's book opens with a short elementary 

 account of the nature of light, of no especial in- 

 terest. Following this comes a very complete 

 account of the work of Seebeck, Becquerel, 

 Poitevin and others. His account of the claims 

 of Hill, the American photographer, are inter- 

 esting, final judgment of the case being left to 

 the reader. 



Full details are given in most cases of the 

 method of preparing the plates, and the reader 

 will find himself strongly tempted to repeat 

 some of these early experiments. 



The third portion of the book treats of the 

 theory of photochromy. The colors of the 

 photochromes had been explained in various 

 ways. Some held that colored oxidation and 

 reduction products were formed while others 

 assumed that the chemical action of the light 

 occurring at the surface, formed a film of vary- 

 ing thickness which showed color precisely like 

 the film of a soap bubble. Zenker effectually 

 demolishes this theory by showing that pro- 

 longed exposure, by increasing the thickness 

 of the film, should change the color, which is 

 not the case. 



He then advances his own beautiful theory, 

 not abandoning the soap film idea, but present- 

 ing it in a wholly new light. He conceives the 

 light waves as penetrating the film and suflTer- 

 ing reflection at the back surface. The re- 

 flected waves interfere with the oncoming 

 waves forming a stationary system, the ether 

 within the film vibrating in nodes, like the 

 string of a musical instrument when sounding 

 a harmonic. He shows us that there will be 

 planes of vibration within the film parallel to 

 the reflecting surface situated half a wave- 

 length apart. In other words the distance 

 between the planes of maximum vibration will 

 depend on the wave-length or color of the 

 light. If the silver is reduced in these planes 

 and not at the nodes (when there is no vibra- 

 tion) we shall have reflecting laminae formed. 



which will act like the upper and lower surface 

 of a soap film and show interference colors. 

 The light most copiously refiected under these 

 conditions will be of a color identical with that 

 of the light which formed the laminae. He 

 describes a number of experiments confirming 

 his theory, but pushes it too far in attempting 

 to explain the color of ordinary objects and the 

 perception of color by the eye in this way. 



His book is on the whole a most excellent 

 resume of the work done up to the time of its 

 publication. 



The appendix, in which the further develop- 

 ment of the subject is treated by E. Tonn, deals 

 chiefly with matters of theoretical interest. The 

 work of Wiener and Lippmann is discussed in 

 connection with the theory of the reproduction 

 of mixed colors. As a matter of fact there 

 have been very few or no developments since 

 the time of Zenker, except along the lines indi- 

 cated by Lippmann, and as no details of this 

 process are given, the appendix is likely to be 

 of interest to the physicist rather than to the 

 photographer. ^ ^ ^^^^ 



BOOKS EECEIVED. 



Grundlinien der anorganischen Chemie. Wilhelm Ost- 



WALD. Leipzig, W. Engelmann. 1900. Pp. six 



-f 795. 18 Marks. 

 Der Gesang der Vogel. Valentin Hackeb. Jena, 



Gustav Fischer. 1900. Pp. vii -f 102. 3 Marks. 

 Symons's British Eainfall, 1899. Compiled by H. 



SowEEBY Wallis. London, Edward Stanford. 



1900. Pp. 251. 10s. 

 Foundations of Knowledge. Alexander Thomas 



Oemond. London and New York, The Macmillan 



Co. 1900. Pp. xxvii + 526. 



SOCIETIES AND AOADEIIIES. 



NEW YOEK ACADEMY OF SCIENCES. 

 SECTION OF GEOLOGY AND MINERALOGY. 



At the meeting on May 21st, Dr. A. A. Julien 

 presided and about twenty persons were pres- 

 ent. Two papers on the rocks of Mexico were 

 presented. The first was by Mr. G. I. Finlay, 

 entitled ' A New Occurrence of Nephaline 

 Syenite and associated Dikes in the State of 

 Tamaulipas, Mexico, with a review of the dis- 

 tribution of these rocks in North America.' 

 The second paper was a ' Contribution to the 



