468 



NA TURE 



[September 5, 1907 



ing an inferior product of little commercial value. 

 There is a reference on p. 384 to " Spanish oil of 

 hops" or "Cretan oil of marjoram," described as 

 obtained from Origainim hirsututn and O. creticum. 

 This appears to refer to the material better known in 

 this country as " Cretan origanum " or " red thvme " 

 oil, which is generally believed to be obtained from 

 Origanum hirttim. The tabular statement of the con- 

 stituents of volatile oils is incomplete in some re- 

 spects; thus, under "basil oil," there is no reference 

 to the terpene ocimene isolated by van Romburgh 

 from this source in igoi ; and the information given 

 under the head of "applications" in these tables is 

 in some cases rather inadequate. 



There is a reference in the list of " errata and 

 addenda " to the recent confirmation by the Philippine 

 Bureau of Science of Trimen and Hentley's statement 

 that " elemi " is obtained from Canarium luzoniciim, 

 but, curiously enough, the generic name is wrongly 

 given as Conarium. 



The arrangement of the subject-matter adopted 

 facilitates reference to the information given regarding 

 most of the products described, but it would have been 

 an advantage if a fuller inde.x had been provided. 



.As a guide to the analysis of conynercial vegetable 

 products this book fully maintains the high standard 

 set in previous editions, and those concerned in its 

 revision are to be congratulated on the satisfactory 

 way in which they have accomplished their work. 



T. A. H. 



SCIENTIFIC ASPECTS OF PHOTOGRAPHY- 

 Investigations on the Theory of the Photographic 



Process. By Dr. S. E. S'heppard and Dr. C. E. 



Kenneth Mees. Pp. .x + 342. (London : Longmans, 



Green and Co., 1907.) Price 6x. 6d. net. 

 TOURING the last four years the authors have been 

 -•->' working with the object of preparing theses for 

 their degrees according to the regulations of the Uni- 

 versity of London, and in this volume they present 

 in order the records of their work by republishing 

 together their communications to several scientific 

 societies. That particular branch of photography that 

 the authors refer to as the photographic process is the 

 exposure, development, fixation, and sensitometry of 

 gelatino-bromide plates— in short, negative making 

 as now understood, but without reference to the after- 

 treatment of the fixed plate by such processes as 

 intensification, or to such collateral matters as the 

 production of developer stains. 



As indicated by the title, the volume is theoretical 

 rather than practical, though the results often have 

 an important practical significance. The subjects are 

 dealt with from the point of view of what is now 

 understood as physical chemistry, and the work is 

 described in the language of that branch of science. 

 This will make the volume probably more acceptable 

 to students interested in photography who have 

 devoted themselves specifically to physical chemistry, 

 but it imposes a serious difficulty in the way of those 

 who have not. This difficulty is increased by the 

 constant use of symbols instead of words in the text. 

 We would suggest the addition of a glossary giving 

 the exact meaning of each of the symbols used. 

 NO. 1975, ^-IL. 76] 



Some of the apparatus used appears to be disadvan- 

 tageously complex. For measuring opacities a spectro- 

 photometer is employed, though dispersion of the light 

 is unnecessary, and appears to be undesirable, for it 

 must add sources of error. The authors use the bright 

 green part of the spectrum, but do not say why they 

 throw away the rest. They recognise the fact that 

 the light transmitted by the silver deposits in films 

 is largely scattered, and that their instrument takes 

 very little cognisance of scattered light. They ap- 

 parently assume that the scattered light generally 

 bears a constant proportion to the whole, but it is 

 very doubtful whether such an assumption is well 

 founded. On certain occasions the authors endeavour 

 to obviate the error due to scattering by making the 

 film itself the practical light-source by means of a 

 diffusing medium, placed in contact with it. Opal 

 glass, which they use, is probably the best diffusing 

 medium available, but the present writer does not 

 call to mind any proof that it is thoroughly effective 

 for such a purpose. Other methods of measurement 

 are well known that do not suffer from these draw- 

 backs. 



The authors deal with many questions that are of a 

 very debatable character, and it in no sense belittles 

 their work to say that they remain debatable. They 

 adopt the " germ " theory of the developable image, 

 but limit the effect of each germ to the grain or 

 nodule of which it forms a part. The " suspicion of 

 a vicious circle in the argument " that the authors refer 

 to in connection with the " molecular strain " theory 

 is, we fear, much more widely applicable than they 

 appear to realise. The authors remark that " it may 

 be said of the physical theories that they shirk a real 

 explanation by treating the phenomena to be explained 

 as a quality of the physical modification of the halide " ; 

 to which one might reply that those who advocate 

 the.chemical theories assume a decomposition of which 

 there is no evidence. And so it remains a matter of 

 opinion as to which is the safer and more useful 

 hypothesis. We make only one other observation, 

 namely, that it seems undesirable to speak of the laiv 

 of constant density ratios, when, as the authors them- 

 selves admit, it is "frequently not followed." When 

 this " law " was first propounded by Messrs. Hurter 

 and Driffield they maintained that it was really a 

 law, which it was very difficult, if possible, to get 

 away from. 



This volume will find a place, which it will worthily 

 fill, in the libraries of all who are interested in the 

 scientific aspects of photography, because it contains 

 the record of a series of carefully conducted experi- 

 ments under stated conditions, and gives copious 

 references to the literature of the subject. C. J. 



OUR BOOK SHELF. 



Surgical Instruments in Greek and Roman Times. 

 By Dr. John Stewart Milne. Pp. xii+187; illus- 

 trated. (Oxford : The Clarendon Press, 1907.) 

 Price 145. net. 

 Both the author and the Clarendon Press are to be 

 congratulated on the issue of this very valuable work — 

 Dr. Milne because it represents work well done, and 

 the Press on account of the successful manner in which 



