Intelligence and Miscellaneous Articles. 235 



as are consistent with the ideas of Faraday and Maxwell. Familiar 

 terms like charge and current have been retained, but their relation 

 to the ethereal medium is made clear as each arises ; by this means 

 the authors have been able to adhere to the order of treatment 

 followed by Joubert. The result is a very satisfactory text-book, 

 with all the advantages that can be imparted to it by teachers of 

 skill and experience. J. L. H. 



Computation Rules and Logarithms, with tables of other useful 



functions. By Silas W. Holman. New York : Macmillan, 



1896; pp. xlv + 73. 

 This is not a Manual such as the ' Manual of Logarithms ' by 

 G. E. Matthews and the ' Examples for practice in the use of seven- 

 figure Logarithms ' by Wolstenholme, nor is it conterminous with 

 the ' Logarithmic Tables ' by Prof. George W. Jones, but it 

 treats the subject more from the point of view of the engineering 

 and scientific student. This is to be expected from the author's 

 position of Professor of Physics at the Massachusetts Institute of 

 Technology. 



The preliminary matter is very clearly put and several practical 

 exercises are worked out. It treats of Rules of Computation (up 

 to date), Logarithms, Antilogs and Cologs, Squares and Square 

 Eoots, Reciprocals, the ordinary Trigonometrical Natural and 

 Log Sines, &c, and Slide Wire Ratios (this will be of use to students 

 of physical chemistry). Eor those who are unacquainted with the 

 terms employed in the previous introduction, definitions and 

 explanations of them are appended. 



The tables are in the main four-place ones, but space is also 

 devoted to five-place logarithms of numbers and of the Trigono- 

 metrical functions. 



Prof. Holman has done his work well, and the many contrivances 

 in working and in the printing of the tables evince him to be a 

 thoroughly practical teacher. We can accord him no higher praise. 



XXX. Intelligence and Miscellaneous Articles. 



BLACK LIGHT. BY M. GUSTAYE LE BON. 



THE recent publication of photographic experiments with light 

 of kathodic origin has decided me to make known, though 

 they are still very incomplete, some researches which I have carried 

 on for the last two years in photography through bodies opaque to 

 ordinary light. The two subjects are very different ; only in their 

 results are there some analogies. 



The following experiments prove that ordinary light, or at any 

 rate some of its radiations, traverses the most opaque bodies without 

 difficulty. Opacity is a phenomenon which only exists for an eye 

 like our own ; if it were somewhat differently constructed, we 

 could see through walls. 



In an ordinary positive photograph-frame place a sensitive plate 

 and above this any ordinary photograph, then above the photograph, 



