1898.] 



on Mngneto-Optic Rotation. 



711 



Fig. 8. 



liglit is sent once along a column of any of those substances with- 

 out any magnetic field, its plane of rotation is rotated just as it 

 is in heavy glass or bisulphide of carbon in a magnetic field. But 

 if the ray, after passing through the 

 column of sugar or quartz, is received 

 on a silvered reflector and sent back 

 again through the column to the start- 

 ing point, its plane of polarisation is 

 found to be in the same direction as at 

 first. Quite the contrary happens when 

 the rotation is due to the action of a 

 magnetic field. Then the rotation is 

 found to be doubled by the forward 



and backward passage, and it can be increased to any required degree 

 by sending the ray backward and forward through the substance, as 

 shown in this other diagram (Fig. 8). 



Thus the rotations in the two cases are essentially different, and 

 must be brought about by different causes. In fact, as was first, I 

 believe, shown by Lord Kelvin, the annulment of the turning in 

 quartz, and the reinforcement of the turning in a magnetic field, pro- 

 duced by sending the ray back again after reflection at the surface of 

 an optically denser medium, points to a peculiarity of structure of 

 the medium as the cause of the turning of the plane of polarisation in 

 sugar solutions and quartz, and to the existence of rotation in the 

 medium as the cause of the turning in a magnetic field. Think of 

 an elastic solid, highly incompressible and endowed with great elas- 

 ticity of shape and of the same quality in different directions — a stiff 

 jelly may be taken as an example to fix the ideas. Now let one 

 portion of the jelly have bored into it a very large number of 

 extremely small corkscrew-shaped cavities, having their axes all 

 turned in the same direction. Let another portion have imbedded in 

 it a very large number of extremely small rotating bodies, spinning- 

 tops or gyrostats in fact, and let these be uniformly distributed 

 through the substance, and have their axes all turned in the same 

 direction. 



Both portions would transmit a plane polarised wave of trans- 

 verse vibration travelling in the direction of the axes of the cavities 

 or of the tops with rotation of the plane of polarisation ; but in the 

 former case the wave, if reflected and made to travel back, would have 

 the original plane of polarisation restored ; in the latter the turning 

 would be doubled by the backward passage. 



To understand this it is necessary to enter a little in detail into 

 the analysis of the nature of plane polarised light. As I have 

 already said, the elastic solid theory may not express the facts of 

 light propagation, but only a certain correspondence with the facts. 

 But its use puts this matter in a very clear way. In a ray of plane 

 polarised light each portion of the ether has a motion of vibration in 

 a line at right angles to the ray, and the direction of this line is the 



