2 Messrs. 0. Wiener and W. Wedding. Magnetic [Dec. 5, 



tigations having "been suggested by Professor Kundt. Experiments on 

 the rotation of the plane of polarisation of light by means of interrupted 

 currents ; like those of Bichat and Blondlot, are here excluded. Since 

 the above-mentioned papers have not been noticed by Mr. Ward, 

 and are hence probably not well known, it may be of interest to re- 

 produce here their essential contents. 



Gouy shows that in a body, which at the same time has the power 

 of double refraction and of rotation, certain vibrations, to which 

 he gives the name " privilegiees," are propagated unaltered. These 

 vibrations play exactly the same part in such bodies as the linear 

 components in, and normal to, the principal secbion in the case of 

 ordinary doubly refracting bodies, and as Fresnel's two circular com- 

 ponents in the case of ordinary rotating bodies. In bodies which at 

 the same time have the power of double refraction and of rotation, these 

 two privileged components of vibration take place in opposite direc- 

 tions in two ellipses, whose major axes are one in the plane of prin- 

 cipal section and the other perpendicular to it. The ratio of the axes 

 and the difference of phase of the components are calculated by Gouy 

 from the constants of double refraction and rotation. 



For the complete solution of the problem of the propagation of 

 light in such a body we only require to resolve any incident vibration 

 into its two privileged components, and to compound them to a single 

 resultant on emergence. The solution of this problem is contained in 

 the above paper by Wiener, which was intended to supply the theore- 

 tical foundation for Wedding's investigation. It was especially im- 

 portant to determine how the rotation, due to the rotational power of 

 the body, is disturbed by double refraction. It was found that the 

 rotation alters periodically with the thickness of the plate, and with 

 strong double refraction may even become negative, that is to say, in 

 the opposite direction to that due to the rotational power. The rota- 

 tion is zero in the neighbourhood of those places where the difference 

 of phase of the linear components, due to double refraction, is a 

 multiple of tt, and not, as Mr. Ward thinks, of tt/2. The general 

 result of Wiener's paper may be quoted as follows : — 



" Herrscht bei der gleichzeitigen Wirkung von Circularpolarisation* 

 und Doppelbrechung die eine vor, so wird die andere theilweise 

 verdeckt. Eine starke Doppelbrechung driickt die Drehung der 

 Circularpolarisation und eine starke Circularpolarisation das Ellip- 

 tischmachen der Doppelbrechung herab." 



Wedding's investigation is of purely experimental nature, and 

 consists of two parts. The first is occupied with magnetic rotation 

 in stressed glass, and completely confirms by experiment the con- 



* By " Circularpolarisation " is understood what we have here called rotational 

 power. 



