704 



Professor Andrew Gray 



[April 29, 



I Lave already referred, and on which almost all the later work on 

 the relation of magnetism to light has been founded. I am permitted 

 by the kindness of the authorities of this Institution to exhibit here 

 the very apparatus which Faraday himself employed, though for the 

 various experiments I have to make it is necessary to actually use 

 another set of instruments. [^Apjparatas shown.'] Before repeating 

 Faraday's experiment, let me describe shortly what I propose to do, 

 and the effect to be observed. 



A beam of plane polarised light is produced by passing white light 

 from this electric lamp through a Nicol's prism. To understand the 

 nature of plane polarised light, look for a moment at this other dia- 





Fig. 1. 



gram (Fig. 1). It represents a series of particles displaced in a certain 

 regular manner to different distances from the mean or equilibrium 

 positions they originally had along a straight line. They are moving 

 in the directions shown by the arrows and with velocities depending 

 on their positions, as indicated by the lengths of the arrows. Suppose 

 a certain interval of time to elapse. The particles will have moved 

 in that time to the positions shown in this other diagram (Fig. 2), on 



Fig. 2. 



the same sheet. It will be seen that the velocities as well as the 

 positions of the particles have altered, but that the configuration is 

 the same as would be given by the former diagram moved through 

 a certain distance to the left. 



Thus an observer looking at the particles and regarding their con- 

 figuration would see that configuration apparently move to the left, 

 and this, it is very carefully to be noted, is a result of the transverse 

 motions of the individual particles. In another interval of time equal 

 to the former, the arrangement of particles will appear to have moved 

 a further distance of the same amount towards the left. 



This transverse motion of the particles, thus shown displaced from 



