M. Pouillet on the Recent Researches o/Prof. Faraday. 331 



that the total effect obtained by passing from the magnetic 

 action which is exerted in one direction to that which is ex- 

 erted in the other, is equal to that which would be produced 

 by a plate of quartz four-tenths of a millimetre in thickness. 

 Such, up to the present time, is the maximum effect which 

 I have been able to obtain. As we have now a means of com- 

 paring the intensities of this force, it will be very easy to see 

 how it will be modified by the different relative positions of 

 the electro-magnet and of the piece of flint-glass. 



The following are the observations I have made with respect 

 to this point : — 



1. If, instead of placing the electro-magnet in contact with 

 the piece of flint-glass, it is removed parallel to itself in the 

 same horizontal plane, and so that the vertical plane separa- 

 ting the two arms corresponds always to the middle of the 

 flint-glass, the action diminishes, but feebly in proportion as 

 the distance increases, so that at the distance of ten centi- 

 metres, it is still a considerable proportion of what it was 

 when it was actually in contact. 



2. If the electro-magnet is again placed in contact, and the 

 piece of flint-glass slid in the direction of the ray of light to 

 subject it to the action of one only of the poles of the magnet, 

 a moment arrives when the action is wholly null ; then, if it 

 is still slid in the same direction, removing it more and more 

 from its primitive position, until it is placed beyond the pole 

 to which it is submitted, the action begins anew ; but then 

 it is contrary to what it was at first. 



These observations appear to lead to three important con- 

 sequences : — 



It first results, that if we consider the unknown action of 

 the magnet on the flint-glass as being produced by attractions 

 and repulsions, the effect is null when the resultant of these 

 attractive and repulsive forces is perpendicular to the direc- 

 tion of the polarized ray ; and it is at its maximum, on the 

 contrary, when this resultant is parallel to the ray. We may 

 thus, from these considerations, form a just idea of the direc- 

 tion in which it acts, for in considering, always hypothetically, 

 the piece of flint-glass as a piece of soft iron, acquiring two 

 poles from the influence of the magnet, the movement of the 

 plane of polarization occurs to the right when the light enters 

 by the south pole, and proceeds from the south to the north 

 pole, and it occurs to the left when the light enters by the 

 north pole ; consequently, whatever be the position of the 

 piece of flint-glass, if two observations are made on it without 

 touching it, and without disarranging the electrical apparatus, 

 but merely turning the optical apparatus to cause the light 



