684 SCIENCE PROGRESS 



are only too often but poorly carried out, and very often detract somewhat 

 seriously from the value of an otherwise good lecture. The most important 

 part of the book is that devoted to illuminants, and it is obvious that that 

 is, after all, the crux of the whole question, as on the illuminant available 

 depends in large part the type of the other apparatus that must be used. 

 Oil lamps, acetylene, and limelight are referred to and described ; but it is to 

 electrical illuminants that the main part of this section is devoted. The 

 electric arc is the most important of these, and the description given and 

 the instructions for working it are exceedingly good. 



There are minor inaccuracies, such, for instance, as where the passing of 

 luminous carbon particles from one carbon to the other is referred to as if 

 it were the actual source of light. This, of course, is not the case, the source 

 being, in the case of continuous current, the crater on the positive carbon. 

 The length of arc is also arbitrarily laid down as being one-eighth of an inch ; 

 but this again would vary with the amount of current and the size of the 

 carbons. 



The types of arc lamps described and figured are apparently by one 

 maker only, and it cannot be said that these are without any good rivals. 

 Whether, too, the right-angled type of electric is only satisfactory when used 

 with small currents is questionable ; in fact, they do prove exceedingly 

 satisfactory with currents as large as twenty and thirty amperes. 



The actual instruction in the use of the arc and directions for calculating 

 the necessary resistance are very lucid, and exactly the sort of information 

 that a lantern user requires. 



The optical part of the equipment is also described, and sufficient of its 

 principles elucidated to enable those who have to use the apparatus to 

 understand what they need do. It is to be regretted that only the old type 

 of lantern body is figured, and no suggestion is made as to the much newer 

 and more convenient type in which the apparatus partakes of the character 

 of an optical bench. 



In general the book may be recommended to all those who have occasion 

 to use the lantern for educational or any other purpose. 



J. E. B. 



BOOKS RECEIVED 



{Publishers are requested to notify prices) 



The Early Mathematical Manuscripts of Leibniz. Translated from the 

 Latin texts published by Emmanuel Gerhardt, with critical and historical 

 notes. By J. M. Child. London and Chicago : The Open Court Pub- 

 lishing Co., 1920. (Pp. V -f 238.) Price $1-50. 



Girolamo Saccheri's Euclides Vindicatus. Edited and translated by George 

 Bruce Halsted, A.M. (Princeton), Ph.D. (Johns Hopkins). London and 

 Chicago : The Open Court Publishing Company, 1920. (Pp. xxx + 246.) 

 Price $2. 



Algebre a Deux Dimensions. Par M. Stuyvaert, Docteur Special en Mathe- 

 •matiques, Correspondant de 1' Academic Royale de Belgique, Professeur 

 a I'Universite de Gand. Gand : Van Rysselberghe et Rombaut, 

 Editeurs (Ancient Librairie E. van Geothem), Rue des Foulons, i, 1920. 

 (Pp. 223.) Price 12.50 fcs. net. , 



The Mathematical Theory of Electricity and Magnetism. By J. H. Jeans, 

 M.A., F.R.S. Formerly Stokes Lecturer in Applied Mathematics in the 

 University of Cambridge, sometime Professor of Applied Mathematics 

 in Princeton University. Fourth Edition. Cambridge : at the Uni- 

 versity Press, 1920. (Pp. vii + 627.) Price 245. net. 



