applied to the Action of Magnetism on Polarized Light. 89 



A and B have been fully investigated by M. Verdet*, who has 

 shown that the rotation is strictly proportional to the thickness 

 and to the magnetizing force, and that, when the ray is inclined 

 to the magnetizing force, the rotation is as the cosine of that in- 

 clination. D has been supposed to give the true relation between 

 the rotation of different rays ; but it is probable that C must 

 be taken into account in an accurate statement of the phenomena. 

 The rotation varies, not exactly inversely as the square of the 

 wave-length, but a little faster; so that for the highly refrangible 

 rays the rotation is greater than that given by this law, but more 

 nearly as the index of refraction divided by the square of the 

 wave-length. 



The relation (E) between the amount of rotation and the size of 

 the vortices shows that different substances may differ in rota- 

 ting power independently of any observable difference in other 

 respects. We know nothing of the absolute size of the vortices; 

 and on our hypothesis the optical phenomena are probably the 

 only data for determining their relative size in different sub- 

 stances. 



On our theory, the direction of the rotation of the plane of 

 polarization depends on that of the mean moment of momenta, 

 or angular momentum, of the molecular vortices ; and since M. 

 Yerdet has discovered that magnetic substances have an effect 

 on light opposite to that of diamagnetic substances, it follows 

 that the molecular rotation must be opposite in the two classes 

 of substances. 



We can no longer, therefore, consider diamagnetic bodies as 

 being those whose coefficient of magnetic induction is less than that 

 of space empty of gross matter. We must admit the diamagnetic 

 state to be the opposite of the paramagnetic ; and that the vor- 

 tices, or at least the influential majority of them, in diamagnetic 

 substances, revolve in the direction in which positive electricity 

 revolves in the magnetizing bobbin, while in paramagnetic sub- 

 stances they revolve in the opposite direction. 



This result agrees so far w r ith that part of the theory of M. 

 Weberf which refers to the paramagnetic and diamagnetic condi- 

 tions. M. Weber supposes the electricity in paramagnetic bodies 

 to revolve the same way as the surrounding helix, while in dia- 

 magnetic bodies it revolves the opposite way. Now if we regard 

 negative or resinous electricity as a substance the absence of 

 which constitutes positive or vitreous electricity, the results will 

 be those actually observed. This will be true independently of 

 any other hypothesis than that of M. Weber about magnetism 



* Annates de Chimie et de Physique, ser.3. vol. xli. p. 3/0; vol. xliii. p. 37. 

 t Taylor's ' Scientific Memoirs/ vol. v. p. 477. 



