1898.] Professor Andrew Gray on Magneto-Optic Rotation. 703 



WEEKLY EVENING MEETING, 

 Friday, April 29, 1898. 



Basil Woodd Smith, Esq. F.R.A.S. F.S.A. Vice-President, 

 in the Chair. 



Professor Andrew Gray, M.A. LL.D. F.R.S. 



Magneto-Optic Botation and its Explanation by a Gyrostatic Medium. 



The action of magnetism on the propagation of light in a transparent 

 medium has been rightly regarded as one of the most beautiful of 

 Faraday's great scientific discoveries. Like most important dis- 

 coveries it was no result of accidental observation, but was the out- 

 come of long and patient inquiry. Guided by a conviction that (to 

 quote his own words) " the various forms under which the forces of 

 matter are made manifest have one common origin," he made many 

 attempts to discover a relation between light and electricity, but for 

 very long with negative results. Still, however, retaining a strong 

 persuasion that his view was correct, and that some such relation must 

 exist, he was undiscouraged, and only proceeded to search for it more 

 strictly and carefully than ever. At last, as he himself says, he " suc- 

 ceeded in magnetising and electrifying a ray of light, and in illuminat- 

 ing a magnetic line of force.*'' 



Faraday pictured the space round a magnet as permeated by what 

 he called lines of force ; these he regarded as no mere mathematical ab- 

 stractions, but as having a real physical existence represented by a 

 change of state of the medium brought about by the introduction of 

 the magnet. That there is such a medium surrounding a magnet we 

 take for granted. The lines of force are shown by the directions which 

 the small elongated pieces of iron we have in iron filings take when 

 sprinkled on a smooth horizontal surface surrounding a horizontal bar 

 magnet, as in the experiment I here make. [Experiment to show field 

 of bar magnet by iron filings.'] 



The arrangement of these lines of force depends upon the nature 

 of the magnet producing them. If the magnet be of horse-shoe shape 

 the lines are crowded into the space between the poles ; and if the pole 

 faces be close together and have their opposed surfaces flat and parallel, 

 the lines of force pass straight across from one surface to the other 

 in the manner shown in the diagram before you. [Diagram of field 

 between flat pole faces.'] 



The physical existence of these lines of force was demonstrated 

 for a number of different media by the discovery of Faraday to which 



