ITHE WAVE THEORY OF LIGHT. 307 

 

 ;o arranged that the length of the sound-wave 

 s about one foot, for one of the chief "open 

 notes." Higher musical notes correspond to greater 

 and greater frequency of vibration, viz., 1,000, 

 2,000, 4,000 vibrations per second ; 4,000 vibra- 

 tions per second correspond to a piccolo flute of 

 exceedingly small length ; it would be but one and 

 a half inches long. Think of a note from a little 

 dog-call, or other whistle, one and a half inches 

 long, open at both ends, or from a little key 

 having a tube three quarters of an inch long, closed 

 at one end ; you will then have 4,000 vibrations 

 per second. 



A wave length of sound is the distance traversed 

 in the period of vibration. I will illustrate what 

 the vibrations of sound are by this condensation 

 travelling along our picture on the screen. Alter- 

 nate condensations and rarefactions of the air are 

 made continuously by a sounding body. When 

 I pass my hand vigorously in one direction, the 

 ir before it becomes dense, and the air on the 

 other side becomes rarefied. When I move it in the 

 other direction these things become reversed ; there 



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