330 M . Pouillet on the Recent Researches of Prof. Faraday. 



there had been interposed, without magnetic action, a lamina 

 of quartz perpendicular to the axis, of a proper thickness, 

 turning to the right in the first case and to the left in the 

 second, it is certain that the equality of the tints would have 

 been re-established by the same movements of the compen- 

 sator. Now, it is known that the effect produced by such a 

 lamina of quartz would have been to turn the plane of pola- 

 rization from right to left, whence it seems very natural and 

 legitimate to conclude, that the flint-glass subjected to the 

 magnetic action has produced the same effect as this lamina 

 of quartz, that is to say, that it has also turned the plane of 

 polarization to the right for one direction of current, and to 

 the left for the contrary direction. This is, in fact, the con- 

 clusion to which Mr. Faraday has come, and he has charac- 

 terized this new action of magnetism upon light, by stating 

 that the magnetism turns the plane of polarization of the 

 luminous ray submitted to its influence under certain condi- 

 tions, and that the direction of this rotation is connected with 

 that of the current. 



Quartz, and the other substances which, of themselves, by 

 their nature or structure have, without the intervention of 

 magnetism, the permanent property of turning the planes of 

 polarization, exert this action with variable intensities on 

 the different elements constituting white light; and there 

 are dispersive powers for this rotation, as there are dif- 

 ferent dispersive powers for refraction. It would be very 

 important to make in this respect some researches upon the 

 substances which acquire this property by the magnetic 

 action, analogous to those very remarkable ones which M. 

 Biot made upon the former. The apparatus which I have 

 used must be very much modified to be adapted to this class 

 of experiments ; it serves to show the phaenomena very di- 

 stinctly, rather than to measure them in their more delicate 

 details. Such an investigation, however, cannot be under- 

 taken with phaenomena so little developed as those which I 

 have obtained; for in such limits they might perhaps be as 

 well explained by partial depolarizations towards the right 

 and left as by the rotation itself of the plane of polarization, 

 which, moreover, would not detract anything, and would 

 perhaps add to their importance. 



As I just stated, the plane of polarization, in that specimen 

 of the flint-glass which gave the most energetic effects, was 

 diverted by the magnetic action as much as it would have been 

 by the action of a plate of quartz two-tenths of a millimetre 

 in thickness ; now, since by changing the direction of the cur- 

 rent, the rotation takes place in an opposite direction, it is seen 



