ACOUSTICS. 71 



of the beam just in front of the fork, serves well to 

 give this double line of light. 



//. Overtone. If the fork be struck about midway of 

 its length, a much higher sound will be heard along 

 with the fundamental. Let the mirror be turning when 

 the fork is struck, and the large sinuous line seen be- 

 fore will now be seen covered with ripples due to the 

 overtone. 



III. Interference. In the place of the mirror at m, 

 place the second fork so that the beam of light from 

 the first will fall upon it, and be reflected to the middle 

 of the screen, having both forks to vibrate perpendicu- 

 larly. Now load the shorter fork with slides until it is 

 nearly in unison with the long fork. When they are 

 both made to vibrate, the line of light upon the screen 

 will be seen to lengthen and shorten with regularity ; 

 at the same time beats will be heard corresponding with 

 the lengthening of the line. These beats may be made 

 to vary in frequency by moving the slides. If the beats 

 are as many as five or six a second, or more, and the 

 second fork be swung upon its vertical axis, the inter- 



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Fig. 40. 



ference may be noted (Fig. 49) ; the swellings corres- 

 ponding to the periods of coincidence, and the con- 

 traction to the periods of interference. 



If the forks are now brought to unison and struck, 

 the resultant figure will depend upon their relative 

 phases. If they have like phases, so that each one 

 reaches its limit at the same instant, the line of light 

 upon the screen will be much elongated, the amplitude 



