624 Notices respecting New Books. 



existence of a back-E.M.F., and the effect of superposing a small 

 alternating current on the continuous current of the arc, being 

 also considered in this connexion. 



An Appendix which is mainly concerned with the mathematical 

 aspect of photometry, and which also contains a list of the most 

 recent papers on the electric arc, forms the concluding section of 

 the work. A very copious index is also appended. 



There is little that we have to find fault with in the book, but 

 it may be pointed out that the chapter on the luminous efficiency 

 of the arc emphasizes the need for a thorough revision of our 

 system of photometric units, which at the present time are in a 

 somewhat chaotic condition. Thus, although the useful term 

 "luminous flux" is employed by Mrs. Ayrton, it is used in the 

 same sense as " quantity of light." It would seem preferable to 

 restrict this latter term, in accordance with Blondel's recommenda- 

 tion, to the time-integral of the luminous flux — a quantity which 

 is of importance in photography, and for which " quantity of light" 

 certainly seems to be the right term. The time seems ripe for the 

 appointment of a Committee of Scientific Experts to deal with the 

 question of nomenclature in photometry. 



Le Phenomene de Kerr et les Phenomenes Electro- Optiques. Par 

 Eugene Neculcea. Paris: C. Naud, 1902. ("Scientia" 

 Series, No. 16.) Pp. x + 92. 



The title of this monograph, which forms one of the most recent 

 additions to the excellent " Scientia " series now being brought out 

 by Messrs. C. JNaud, is somewhat ambiguous, if not misleading. 

 By " Kerr's phenomenon " we generally mean the rotation of the 

 plane of polarization due to the reflexion of polarized light from 

 the pole of a magnet. This, however, is not the effect which forms 

 the subject of the present memoir. The author studies another 

 electro-optic effect, also discovered by Kerr — the double refrac- 

 tion produced by electric polarization of the medium. The very 

 existence of this effect has been doubted by some physicists, who 

 attributed the results obtained by Kerr to purely secondary causes, 

 and not to electric polarization of the medium. In the light ot' 

 more recent experiments, however, there seems to be no room for 

 any further doubt, and the excellent resume of both the experi- 

 mental and theoretical investigations bearing on this subject 

 contained in the present volume will be found most useful by 

 physicists. The author gives an exhaustive bibliography of the 

 subject, and references to various physical text-books in which it 

 is dealt with. The book itself is divided into three parts : Part I. 

 deals with experiments; Part II. with theory; and Part III. is 

 devoted to theoretical considerations regarding the possible existence 

 of a phenomenon analogous to the Zeeman effect in a magnetic 

 field. 





