268 KELVIN 



and the vibrations of light. Here is the fixed appearance 

 of the particles when displaced but not in motion. You 

 can imagine particles of something, the thing whose mo- 

 tion 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 dia- 

 gram 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 repre- 

 sents a particle of the luminiferous ether, moving at the 

 greatest speed when it is at the middle position. 



You see the two modes of vibrations, 3 sound and light 

 now moving together; the travelling of the wave of con- 

 densation and rarefaction, and the travelling of the wave 

 of transverse displacement. Note the direction of propa 

 gation. Here it is from your left to your right, as you 

 look at it. Look at the motion when made faster. We 

 have now the direction reversed. The propagation of the 

 wave is from right to left, again the propagation of the 

 wave is from left to right; each particle moves perpen- 

 dicularly to the line of propagation. 



I have given you an illustration of the vibration of sound 

 waves, but I must tell you that the movement illustrating 

 the condensation and rarefaction represented in that mov- 

 ing diagram are necessarily very much exaggerated, to 

 let the motion be perceptible, whereas the greatest con- 

 densation in actual sound motion is not more than one 

 or two per cent, or a small fraction of a per cent. Except 

 that the amount of condensation was exaggerated in the 

 diagram for sound, you have in the chart a correct rep- 

 resentation of what actually takes place in sounding the 

 low note C. 



On the other hand, in the moving diagram representing 

 light waves what had we? We had a great exaggeration 

 of the inclination of the line of particles. You must first 



a Showing two moving diagrams, simultaneously, on the screen, one 

 demoting a wave motion of light, the other a sound vibration. 



