X891.] The Faraday Centenary, 467 



tions, systematised and expressed in the form of mathematical law 

 by Sir Wm. Thomson, was where the ball would go. [This was 

 illustrated by experiment.] 



The next discovery of Faraday to which he proposed to call 

 attention was one of immense significance from a scientific point of 

 view, the consequences of which were not even yet fully understood or 

 developed. He referred to the magnetisation of a ray of light, or 

 what was called in more usual parlance the rotation of the plane of 

 polarisation under the action of magnetic force. It would be hope- 

 less to attempt to explain all the preliminaries of the experiment to 

 those who had not given some attention to those subjects before, and 

 he could only attempt it in general terms. It would be known to 

 most of them that the vibrations which constituted light were 

 executed in a direction perpendicular to that of the ray of light. By 

 experiment he showed that the polarisation which was suitable to pass 

 the first obstacle was not suitable to pass the second, but if by 

 means of any mechanism they were able after the light had passed 

 the first obstacle, to turn round the vibration, they would then give 

 it an opportunity of passing the second obstacle. That was what 

 was involved in Faraday's discovery. [Experiment.] As he had 

 said, the full significance of the experiment was not yet realised. A 

 large step towards realising it, however, was contained in the 

 observation of Sir Wm. Thomson, that the rotation of the plane of 

 polarisation proved that something in the nature of rotation must be 

 going on within the medium when subjected to the magnetising 

 force, but the precise nature of the rotation was a matter for further 

 speculation, and perhaps might not be known for some time to 

 come. 



When first considering what to bring before them he thought, 

 perhaps, he might include some of Faraday's acoustical experiments, 

 which were of great interest, though they did not attract so much 

 attention as his fundamental electrical discoveries. He would only 

 allude to one point which, as far as he knew, had never been noticed, 

 but which Faraday recorded in his acoustical papers. " If during a 

 strong steady wind, a smooth flat sandy shore, with enough water on 

 it, either from the receding tide or from the shingle above, to cover 

 it thoroughly, but not to form waves, be observed in a place where 

 the wind is not broken by pits or stones, stationary undulations will 



be seen over the whole of the wet surface These are not 



waves of the ordinary kind, they are (and this is the remarkable 

 point) accurately parallel to the course of the wind." When he first 

 read that statement, many years ago, he was a little doubtful as to 

 whether to accept the apparent meaning of Faraday's words. He 

 knew of no suggestion of an explanation of the possibility of waves 

 of that kind being generated under the action of the wind, and it was, 

 therefore, with some curiosity that two or three years ago, at a French 

 watering-place, he went out at low tide, on a suitable day when there 

 was a good breeze blowing, to see if he could observe anything of 



