714 



Professor Andrew Gray 



[April 29, 



propagation for the same direction of motion of the particles is the 

 opposite of that just specified. The direction of propagation remains 

 therefore the same when the direction of motion and the helical 

 arrangement of the particles are both reversed. All this can be 

 made out from the diagram. Fig. B shows part of a right-handed 

 arrangement of j)articles corresponding to the opposite arrangement 

 of Fig. A ; and if the particles have the motions shown at the bottom 

 of the diagram, the propagation will be for both in the same direction 

 from the bottom to the top. 



Fig. 12. 



In Fig. 10 we suppose the periods equal and also the wave- 

 lengths, the distance along the axis from particle 1 to particle 9. 

 The combination of the circular motions A and B gives rectilinear 

 motion ; the combination of the wave motions of Figs. A and B 

 gives a plane polarised wave, the plane of polarisation of which does 

 not change in position. If, however, while the periods were equal, 

 the wave-lengths were unequal, as shown in this other diagram (Fig. 

 12), the plane of polarisation would rotate, as shown by the lines 

 drawn across the paths in the figure on the right,for the circular motions 

 of particles in the longer wave would gain on those in the shorter. 



