its Circular Comj)onents in the Faraday "Effect. 465 



circularly, elliptically, or plane polarized. When they were 

 circularly polarized, the interference-bands from sodium light 

 were displaced in one direction when the magnetic field was 

 thrown on, and in an opposite direction when the field was 

 reversed. The displacement for a reversal w T as *1355 of a 

 band, while the calculated value from the total rotation of 

 the plane of polarization of 49° 20' on the above principle 

 was *137. Righi, Becquerel, and Cornu had also made 

 similar observations. Their investigations, however, were 

 confined to circular vibrations. These results admit of 

 different interpretations. The displacement may have been 

 brought about by a relative difference in velocity of the two 

 opposite circular rays, or the same effect may have been 

 produced by a relative change in the phase of the tw T o com- 

 ponents, or both factors may together have entered into the 

 phenomenon. The observed displacement indicated that the 

 circular vibration corresponding to the direction of the 

 Amperian currents of the field had received a relative 

 acceleration of velocity or of phase over the other component. 

 Experiments were further made to determine whether 

 vibrations of varying ellipticity were also relatively 

 accelerated in velocity and in phase. A similar displace- 

 ment of the bands took place, but they became less and less 

 sharply defined as the ellipticity increased. This indicated 

 merely a rotation of the principal axes of the elliptical 

 vibrations, and not a relative acceleration of the vibrations as 

 a w r hole, since in that case the bands would have remained 

 distinct instead of becoming partially obliterated. ]f the 

 ellipticity had been increased until each ray became plane- 

 polarized, and the relative rotation had been 90°, the bands 

 would evidently have disappeared entirely, giving a uniformly 

 illuminated field. These results, representing the action on 

 li^ht in all modes of vibration, must include that for natural 

 light. They do not, however, settle the fact whether the 

 medium can transmit a disturbance by circular vibrations 

 only. It is true that there was a relative acceleration for 

 circular vibrations, and that this would have produced the 

 effects observed for the elliptical vibrations if we suppose 

 them separated into their circular components, but a direct 

 rotation of their principal axes would have produced the 

 same thing also. It became necessary, then, to apply the 

 direct methods of refraction to the problem. 



Fresnel's combination of right- and left-handed quartz 

 prisms could not be imitated for magnetic substances, as no 

 two substances could be found of opposite rotary power 

 having the same index of refraction and sufficient transparency 



