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POPULAR SCIENCE REVIEW. 
pages the whole subject is discussed, while Dr. Maddox’s name is never 
once mentioned, although he is undoubtedly the first micro-photographer 
that England has produced. Then the description of the apparatus to be 
employed is evidently written by one who never endeavoured to take a 
microscopic photograph in his life. Still, however, we should remember 
that the author addresses the general and not the scientific public. And 
bearing that in mind, we would refer in conclusion to his chapter on 
u photography in natural colours.” In this we have explained to us at 
some length the many difficulties that surround the subject, and the many 
partially successful efforts that have been made in this direction. It 
certainly does seem wonderful that an attempt in this way was made 
as long ago as 1810 by Professor Seebeck, of Jena, who found that chloride 
of silver took on the natural colours of the spectrum. Of course the main 
difficulty has been that of discovering a mode of fixing the colours, none 
being yet known. There is still work being done in this direction by M. 
Poitevin, of Paris ; Herr D. Zencker, of Berlin ; and Mr. Simpson, of London ; 
and we may hope that future workers will discover a method of fixing these 
coloured representations. “ The first attempts in uncoloured photography 
also failed for want of a fixing medium, which was only discovered seventeen 
years later by Herschel.” 
A MANUAL OF ELECTRICITY.* 
E LECTRICITY has now become so important an agent and so gener- 
ally employed in the arts and manufactures, that there certainly is re- 
quired a work dealing with the whole subject ab initio, which those who desire 
to understand the subject might take up and read with advantage. We fear, 
however, that Mr. J. T. Sprague is hardly the person who could write such 
a book. It is a great mistake to suppose that a minute practical acquaint- 
ance with a subject is all that is requisite for a teacher j and we have no 
doubt that if this fact were once thoroughly recognised our publishers would 
have infinitely less to do than at present. But, unhappily, it too often 
happens that a man who thoroughly understands a subject is selected to 
teach it, without the possession of the slightest knowledge of how to impart 
his ideas. Now, such a case is that of the author of the present book. We 
do not doubt that Mr. J. T. Sprague is thoroughly qualified as a telegraphic 
engineer ; but we certainly cannot recognise him as a teacher of science. 
His opening sentence is of itself quite sufficient to show that he is utterly 
unqualified to impart knowledge. What will any of our modern philosophers 
say to the following as the commencement of a work which proposes to 
teach practical science P — u When we endeavour to analyse and simplify our 
ideas of the universe of which we form part, we find that we reduce it to 
three distinct conceptions, which we define as essences or entities, viz. Matter, 
* u Electricity : its Theory, Sources, and Application.” By J. T. Sprague, 
Member of the Society of Telegraphic Engineers. London : Spon. 1875. 
