414 ScientifiG Proceedings, Royal Dublin Society. 



displacement, wliile the direction of polarisation is the direction of 

 electric displacement. The direction of magnetic displacement is 

 the direction of the plane of polarisation of the ray. 



'From the fact that we can represent a medium in three dimen- 

 sions, such that its equations of motion are the same as those 

 Maxwell has shown must exist in the ether, it is at once evident 

 that it would he possible to reproduce all the phenomena of the 

 reflection and refraction of light and of polarisation of light, and 

 that not merely in ordinary but in crystalline media, so that it 

 would be even possible to reproduce external and internal conical 

 refraction and the other peculiarities of the wave surface. Two 

 related cases may well be noticed — they are those of rotatory 

 polarisation by crystalline and magnetised media respectively. 

 The first would obviously be obtained by giving a twist to the 

 planes containing the wheels, a plane polarised ray would then have 

 its plane of polarisation twisted during its passage through the 

 medium in the same way as the plane of polarisation of a ray of 

 light is twisted in passing along the axis of a crystal of quartz. 

 The phenomena of magnetised media cannot be so easily repro- 

 duced. It would evidently require that something should be 

 rotating in the field besides the wheels, and that it and the wheels 

 should be so connected that a rotation of the wheels should tend to 

 change the axis of rotation of this rotating something, which, 

 reacting on the wheels, would change their direction of rotation. 

 Mechanism that did this would reproduce the phenomena of the 

 rotatory polarisation of magnetised media. It is worth while 

 remarking how it is necessary, in order to represent this pheno- 

 menon, to take into account a change produced by the wave in 

 something besides the wheels, &c., which then reacts on the wave 

 propagation, so that the action is altogether secondary when com- 

 pared with a peculiarity of the bands or wheels themselves existing 

 in any region independently of the wave passing through it. This 

 latter state is analogous to the state of the ether inside a body pro- 

 duced by the presence of the matter that causes its refractive index 

 and specific inductive capacity to differ from unity. The disper- 

 sion of waves of different rates of vibration is, on the other hand, 

 a secondary action, as is illustrated by several theories of dispersion 

 where the action of the matter in the wave propagation is due to a 

 reaction of the matter during the wave propagation, and not a 



