THE WAVE THEORY OF LIGHT. 309 



just as you see here. But that process shoots 

 forward at the rate of about one thousand feet 

 per second ; the exact period of the motion 

 being 256 vibrations per second for the actual 

 case before you. 



Follow one particle of the air forming part of 

 a sound wave, as represented by these moving 

 spots of light on the screen ; now it goes down, 

 then another portion goes down rapidly ; now 

 it stops going down ; now it begins to go up ; 

 now it goes down and up again. As the 

 maximum of condensation is approached it is 

 going up with diminishing maximum velocity 

 The maximum of rarefaction has now reached 

 it, and the particle stops going up and begins 

 to move down. When it is of mean density 

 the particles are moving with maximum velocity, 

 one way or the other. You can easily follow these 

 motions, and you will see that each particle moves 

 to and fro and the thing that we call condensation 

 travels along. 



I shall show the distinction between these 

 vibrations and the vibrations of light. Here is the 



