1898.] 



on Magneto-Optic Rotation. 



705 



their equilibrium positions, represents the vibration of the medium 

 which is the vehicle of light, and the right to left motion of the con- 

 figuration of particles is the wave motion resulting from that vibration, 

 I do not say that the medium is thus made up of discrete particles, 

 or that the different portions of it vibrate in this manner, but there 

 £•8 undoubtedly a directed quantity transverse to the direction in 

 which the wave is travelling, the value of which at different points 

 may be represented by the displacements of the particles, and which 

 varies in the same manner, and results, as here shown, in the propaga- 

 tion of a wave of the quantity concerned. 



In fact we have here a representation of a wave of plane polar- 

 ised light. The directions of vibration are right lines parallel at all 

 points along the wave. Ordinary light consists of vibrations the 

 directions of which are not parallel if rectilinear, and each vibration 

 is therefore capable of being resolved into two in directions at right 

 angles to one another. The Nicol's prism in fact splits a wave of 

 ordinary unpolarised light into two waves, one in which the vibra- 

 tions are in one plane containing the direction in which the light is 

 travelling, the other in a plane containing the same direction, but at 

 right angles to the former. One of these waves is stopped by the film 

 of Canada balsam in the prism and thrown out of its course, while the 

 other wave is allowed to pass on undisturbed. 



If the wave thus allowed to pass by one Nicol's prism be received 

 by another, it is found that there are two positions of the latter in which 

 the wave passes freely through the second prism, and two others in 

 which the wave is stopped. The prism can be turned from one posi- 

 tion to another by properly placing it and then turning it round the 

 direction of the ray. It is found that if the prism be thus turned 



Fig. 



from a position in which the light is freely transmitted, we come after 

 turning it through 90"" to a position in which the light is stopped, 

 and that if we go on turning tlirough another angle of 90° a position 



