THE 

 LONDON, EDINBURGH and DUBLW 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



SUPPLEMENT to VOL. XXXIV. THIRD SERIES. 



LXXI. On Circular Magiietic Polarization, 

 By M. A. Bertin*. 



SINCE the discovery of Prof. Faraday, circular magnetic 

 polarization has formed the subject of several important 

 investigations ; but nevertheless there still remain some con- 

 ditions of the phaenomenon to discuss, some conclusions to 

 verify, some obscure points to clear up. I have attempted to 

 do this, aided by the valuable assistance of MM. Pouillet and 

 Edmund Becquerel, who have kindly placed at my disposal 

 the apparatus they made use of in their researches on this 

 subject. 



The method of experimenting is now too well known to 

 require description. I shall only say that all the numbers 

 referred to in this paper represent the total rotation produced 

 in the plane of polarization by a change in the direction of the 

 current. It is this total rotation which I have always mea- 

 sured, as it is immediately presented in the experiment; be- 

 cause, corresponding to a more intense phaenomenon, it is for 

 that very reason measured with greater accuracy ; and lastly, 

 because it is independent of the determination, always very 

 uncertain, of the zero, that is to say of the position of the 

 analyser, for which the light was extinguished before the pas- 

 sage of the current. 



As is seen, the measurement of the rotation results from the 

 determination of two planes of polarization, or of two tints of 

 passage ; each of these observations being liable to an error of 

 \ degree, it is necessary to admit the possibility of an error of 

 \ degree in the rotation, which moreover also is subject to the 

 influence of small irregularities in the transmission of the cur- 

 rent by the commutator. The variations which affect the cur- 



* From the Annates de Chimie et de Physique, vol. xxiii. p. 5. 

 Phil. Mag, S. 3. No. 232. Suppl, Vol. 34. 2 I 



