346 SCIENCE PROGRESS 



has the advantage of being easily workable. Quartz should find a place with 

 sulphur among the best insulators, and reference might well have been made 

 to the ageing and accompanying deterioration of most insulators. 



In his treatment of electroscopes the author mentions that weight for weight 

 aluminium leaves are more durable than gold. He does not tell us that when 

 sensitiveness is aimed at there is little to choose between the two so far as 

 weight is concerned, for although gold leaf can be obtained thinner than 

 aluminium leaf, its lesser thickness is counterbalanced by greater density. With 

 the ordinary methods of mounting, the thinner gold leaf holds a considerable 

 advantage by reason of its greater flexibility near the hinge. 



All these are minor blemishes, and the volume, for its size, takes a remarkably 

 comprehensive survey of the subject in its most recent phases. Mr. Raffety 

 writes easily and pleasantly, there is a good deal that is meritorious, and this book 

 will, no doubt, find readers who will be encouraged to ask for something more. 



G. W. C. Kaye. 



The Photography of Coloured Objects. By C. E. Kenneth Mees, D.Sc. 

 [Pp. vi + 69] (Croydon : Wratten & Wainwright, Ltd., 1909. Price is. 

 net.) 



As is generally known, there is a great difference between the sensitiveness of 

 the eye and of the ordinary photographic plate to light of different colours. By 

 treating a plate with certain dyes its lack of sensitiveness can be partially 

 corrected ; but for it to attain the same relative sensitiveness a colour screen 

 must be used in addition to diminish the excessive action of the light of the blue 

 end of the spectrum. 



In the first three chapters of this little book, Dr. Mees gives clearly and 

 concisely a general introduction to the principles underlying the photography 

 of coloured objects, and in the subsequent chapters discusses the application 

 of these principles to the more important branches of work. The chapters 

 on "photography of coloured objects for reproduction," "landscape photography," 

 and " portraiture " are contributed by authorities in these particular branches of 

 photography. 



The author also explains how, by the use of appropriate colour-screens and 

 panchromatic plates, it is possible to overcome difficulties such as the rendering 

 in monotone of colour contrasts in the copying of pictures and in microphotography, 

 also the photography of subjects having a uniform colour, such as articles of 

 furniture. A method of suppressing certain colours is described, the example 

 being the entire elimination of red ink corrections from a typewritten sheet. The 

 great advantage that can be effected by the special treatment of these particular 

 cases is strikingly shown by the illustrations. Finally, a chapter is devoted to 

 the theory underlying the processes of three-colour photography. 



This work will no doubt prove both interesting and instructive to all concerned 

 in the subject of photography. 



A. E. Andrews. 



The Theory of Valency. By J. Newton Friend. [Pp. ix 4- 1S0.] (London : 

 Longmans, Green & Co., 1909. Price 5^. net.) 



This treatise, which is one of the series of chemical text-books edited by Sir 

 William Ramsay, contains a concise and very readable summary of the views 

 which have been held at various times concerning valency ; it is moreover the only 



