M. A. Benin on Circular Magnetic Polarization. 485 



to' 



it has the same direction as the current which, according to 

 Ampere, would be established under the influence of an 

 electro-magnet in a piece of soft iron inserted in the place of 

 the substance experimented with. 



It may not be without interest to connect this general law 

 with the theory which M. Fresnel has given of circular pola- 

 rization. After having shown that a ray of light polarized at 

 right angles can be decomposed into two others polarized 

 circularly in a contrary direction, and vice versa, it sufficed 

 for him to suppose that a plate of quartz cut perpendicular to 

 the axis has the property of allowing to pass, with unequal 

 velocities, rays polarized circularly in one or other direction, 

 and all the rotatory properties of the quartz followed from 

 this simple supposition. Let us admit, in like manner, that 

 the presence of the electro-magnet, or of a circular current, 

 which is the same thing, communicates to transparent bodies 

 the property of allowing the circularly polarized rays to pass 

 more easily, the luminous molecules of which rotate in the 

 same direction as the current, and the general law which I 

 have announced will result from this simple hypothesis. 



To be convinced of the accuracy of this law, it suffices to 

 observe, as I have done, the direction of the rotation for all 

 positions of the glass, or of the transparent substance in gene- 

 ral, in relation to the current. 



First. If the glass is placed between the two poles of the 

 electro-magnet f two cases may occur. 



Either these two poles are directly opposed to the glass, as is 

 the case in RuhmkorfF's apparatus: then there is no doubt as 

 to the direction of the current. For instance, on viewing the 

 glass from the surface which touches the south pole, we see 

 that it is subject to a current proceeding from left to right ; 

 and the rotation observed is, in effect, in this direction ; — it 

 changes direction with the current. Or the two branches of 

 the electro-magnet, instead of being in the axis of the glass, 

 are perpendicular to it, which takes place in the iron horse- 

 shoe electro- magnet, and then the currents are oblique in re- 

 lation to the glass, or parallel to its axis. But the effect ought 

 always to be the same as above; for a piece of soft iron in- 

 serted in the place of the glass would assume the same poles 

 in both cases: only the intensity would be much less; and to 

 increase it, it would be necessary to bring the polar axes 

 nearer to that of the glass. This effect is produced by M. 

 Becquerel's terminations. 



Secondly. In a straight electro-magtietic helix traversed in 

 the direction of its axis by a ray of light, the current is in 

 the same direction the whole length; consequently the rota- 



