THE WAVE THEORY OF LIGHT 267 



other side becomes rarefied. When I move it in the other 

 direction these things become reversed; there is a spread- 

 ing out of condensation from the place where my hand 

 moves in one direction and then in the reverse. Each 

 condensation is succeeded by a rarefaction. Rarefaction 

 succeeds condensation at an interval of one-half what we 

 call " wave-lengths." Condensation succeeds condensation 

 at the full interval of a wave-length. 



We have here these luminous particles on this scale, 1 rep- 

 resenting portions of air close together, more dense; a 

 little higher up, portions of air less dense. I now slowly 

 turn the handle of the apparatus in the lantern, and you 

 see the luminous sectors showing condensation travelling 

 slowly upwards on the screen; now you have another 

 condensation making one wave-length. 



This picture or chart represents a wave-length of four 

 feet. It represents a wave of sound four feet long. The 

 fourth part of a thousand is 250. What we see now of 

 the scale represents the lower note C of the tenor voice. 

 The air from the mouth of a singer is alternately con- 

 densed and rarefied just as you see here. But that proc- 

 ess 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 dimin- 

 ishing 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 par- 

 ticles 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 



1 Alluding to a moving diagram of wave motion of sound produced by a 

 working slide for lantern projection. 



