1889.] on Optical Torque. 497 



out the interference fringes ; but on magnetising one of the two 

 pieces of heavy-glass, or on magnetising the two in opposite senses, 

 the interference bands can be made to reappear. It is in this way 

 that Professor Sohncke's experiment — hardly suitable for a lecture 

 theatre — was performed. It is in this way that we establish upon an 

 experimental basis the fact that light itself, and not merely the plane 

 of its polarisation, experiences an optical torsion when subjected to 

 those forces which, whether crystalline, molecular, or magnetic, exert 

 upon it an optical torque. 



[S. P. T.] 



