Littell's Living Age/Volume 133/Issue 1715/Rotatory Magnetic Polarization

From The Academy.


In 1845 Faraday discovered that a powerful magnet exercises an action on many substances placed between its poles, such that if a ray of plane-polarized light traverses them in the direction of the line of the poles, the plane of polarization is deflected through a certain angle. The direction of displacement — according to the further experiments of Verdet — depends upon whether the medium between the poles is a diamagnetic or a paramagnetic substance. M. Henri Becquerel has lately presented to the French Academy an important memoir in which he endeavors to find some relation between the rotatory magnetic polarization of a substance and its refractive index, and has with this object investigated the optical properties of a great number of substances of high refracting power which have never before been examined from this point of view. It appears from the numbers given that the rotatory magnetic polarization increases with the refractive index, but much more rapidly than in a simple ratio. With respect to solutions of salts it appears that the rotation increases with the concentration, and, moreover, that anomalous rotatory dispersion is accompanied by negative magnetic rotation. In connection with this subject we may mention some observations which have been made by Mr. G. F. Fitzgerald, on the subject of Dr. Kerr's experiment. It will be remembered that at the last meeting of the British Association Dr. Kerr announced the discovery that the plane of polarization of a ray of light reflected from the polished pole of a magnet is rotated. Mr. Fitzgerald (Proc. Royal Soc., xxv. 441) offers an explanation of this remarkable fact by reference to the action of a diamagnetic transparent substance in a powerful magnetic field on a ray of plane-polarized light passing through it. The plane-polarized ray may be regarded as the resultant of two circularly-polarized rays, one right and the other left handed, the former of which has a higher refractive index for the medium than the latter, if the rotation is towards the right, and a less, if the rotation is towards the left. Applying this consideration to the case of reflection of a polarized ray from the reflecting surface of a south magnetic pole, Mr. Fitzgerald arrives at the conclusion that the reflected beam is elliptically polarized, the major axis of the ellipse making a small angle to the right of the plane of incidence. This theoretical result was confirmed by a direct experiment, and appeared also to be in harmony with Dr. Kerr's experiments. We understand that Dr. Kerr has obtained some further results in addition to those which he communicated to the British Association. We shall be glad when these are published, so that we may see their bearing on Mr. Fitzgerald's conclusions.