170 Notices respecting New Books. 



independently of the obvious misprints. We are not able to give a 

 general equation for that action ; but carbonic acid would certainly 

 be formed ; it may probably be affirmed that chromic acid would 

 be reduced to chromic oxide, which would then combine with 

 organic matter to form an insoluble compound. Where changes 

 are possibly complicated, and certainly obscure, it would be better 

 to explain them in somewhat general terms than attempt to tie 

 them down in an equation. 



The equation on page 141, for the action of gold chloride on 

 sodium acetate in the toning-bath, can scarcely be admitted to be 

 correct on the evidence that the solution becomes acid. This acid 

 reaction may be accounted for in a variety of ways, one of the 

 least probable being that trichloracetic acid is formed under these 

 circumstances. 



In the chapters on dry-plate and emulsion processes, and the 

 like, the author has, it seems to us, rightly apprehended his task. 

 He does not overload his book by an account of all possible modi- 

 fications of these methods, but selects one or two approved typical 

 ones, and describes and explains them with some fulness. 



The principal methods of printing with inorganic salts other than 

 silver are given, a branch of photography which, we think, has not 

 received the attention it deserves. Here we would remark that in 

 two or three cases, for instance in the account of Willis's aniline 

 process, by giving the strengths of solutions used he would make his 

 general descriptions not less readable, and at the same time convert 

 it into a workable set of instructions. 



In the account of the methods of printing in permanent pig- 

 ment, and of the photomechanical processes, Captain Abney also 

 preserves a right balance. There is a very useful chapter on lenses, 

 and also one on the most approved forms of apparatus. 



Chapter 31 is devoted to w T hat may be called the aesthetics of 

 photography, — to elucidating the principles which should guide the 

 photographer in endeavouring to produce artistic pictures. A 

 number of illustrations of such subjects by well-known photogra- 

 phers are given ; and the minute analyses of the elements in these 

 which go to produce the artistic effect are both ingenious and in- 

 structive. As something more than technical skill in painting is 

 required to bring forth the highest results, so also in photography : 

 although the process is so largely mechanical, technical profi- 

 ciency alone will not produce satisfying results ; a careful obser- 

 vance, however, of the principles laid down in this chapter will lead 

 to the avoidance of the crude effects so often met with in photo- 

 graphy, and so displeasing to the true lover of art. 



The last chapters of the work are among the most valuable, as 

 they are the most characteristic; they refer to the more purely 

 scientific applications of photography, which the author has himself 

 done so much to promote. Here are dealt with the subjects of acti- 

 nometry, of photo-spectroscopy, of celestial and microscopic pho- 

 tography, and the miscellaneous applications (such as the automatic 

 registration of meteorological and other physical phenomena). These 



