3io POPULAR LECTURES AND ADDRESSES. 



fixed appearance of the particles when displaced 

 but not in motion. You can imagine particles of 

 something, the thing whose motion constitutes 

 light. This thing we call the luminiferous ether. 

 That is the only substance we are confident of in 

 dynamics. One thing we are sure of, and that is 

 the reality and substantiality of the luminiferous 

 ether. This instrument is merely a method of 

 giving motion to a diagram designed for the 

 purpose of illustrating wave motion of light. I 

 will show you the same thing in a fixed diagram, 

 but this arrangement shows the mode of motion. 



Now follow the motion of each particle. This 

 represents a particle of the luminiferous ether, 

 moving at the greatest speed when it is at the 

 middle position. 



You see the two modes of vibration, 1 sound and 

 light now moving together ; the travelling of the 

 wave of condensation and rarefaction, and the 

 travelling of the wave of transverse displacement. 

 Note the direction of propagation. Here it is from 



1 Showing two moving diagrams, simultaneously, on the screen, 

 one depicting a wave motion of light, the other a sound vibration. 



