1889.] on Optical Torque. 475 



magneto-optic rotation in these substances. H. Becquerel has ex- 

 tended them to gases, and has shown how the magnetism of the earth 

 rotates the plane of j)ohirisation of the light which, previously j)^*l^i'" 

 ised by reflection from the aerial particles which give the sky its 

 blue tint, passes earthward through the oxygen of the air. 



Other experimenters have dealt with the rotatory effects (whether 

 crystalline, molecular, or magnetic) in relation to lights of different 

 colours, and have studied the dispersion which arises from the 

 greater actual angle of optical torsion which is produced ujDon weaves 

 of short wave-length (violet and blue) than that which is })roduced 

 under the influence of equal rotatory forces upon the waves of longer 

 wave-length (red and orange). It has also been demonstrated that 

 the plane of polarisation of waves of invisible light, whether those of 

 the infra-red, or those of the ultra-violet species, if they have been 

 previously polarised, can be rotated just as can that of waves of 

 visible light. 



In 1877, Dr. Kerr, of Glasgow, discovered a point w'hich Faraday 

 had sought for, but fruitlessly — namely, that in the act of reflection 

 at the pole or surface of a magnet, there is a rotation of the plane of 

 polarisation of light. This discovery was completed in 1884 by 

 Kundt, of Strasburg, by the further demonstration, also dimly fore- 

 seen by Faraday, that a magneto-optic rotation of the plane of polari- 

 sation is caused by the passage of previously polarised light through 

 a normally magnetised film of iron so thin as to be transparent. 



Lastly, in this brief enumeration, we were shown a month ago, by 

 Oliver Lodge, how the magnetic impulses generated by the raj^id 

 oscillatory discharges of the Leydeu jar can produce corresponding 

 raj)id oscillatory rotation in the plane of polarisation of the waves 

 of previously polarised light. 



You will not have failed to notice the cumbrous j^hrase which, 

 whether in speaking of the purely optical effects (of quartz, or sugar, 

 or turpentine), or in speaking of the magneto-optic effects of more 

 recent discovery, I have employed to connote a very simple fact. 

 You may have wondered that any lover of simple English speech 

 should indulge in such sesquipedalian words. 



Of course, at this period of the nineteenth century it is no longer 

 open to debate that light consists of waves. The plaue of polarisation 

 of the waves of light is the plaue of polarisation of the light itself. 

 The rotation of the plane of polarisation is the rotation of the polar- 

 ised waves, and therefore of the polarised light itself. Yet 1 must 

 draw attention to the fact that in all the array of discoveries which 

 I have enumerated, that which had been observed was the rotation — 

 whether by crystalline, molecular, or magnetic means — not of 

 natural light, but of light which had by some means been previously 

 polarised. It was not known to Arago or to Biot, to Fresnel, to 

 Faraday, nor even to Spottiswoode or to Maxwell, that natural 

 impolarised light could be rotated. They may have inferred so, but 

 it was not in their time even demonstrable that a beam of circularly- 



YoL. XII. (No. 83.) 2 K 



I 



