TONES BY INFLUENCE. 193 



tions iii the air produced by the primary tone. "We have 

 already spoken of this under the head of resonance ; l and 

 sounds produced in this way are properly called tones by in- 

 fluence. 



It is evident that the mechanism of the production of 

 tones by influence cannot be neglected in studying the physi- 

 ology of audition. "We have, as an important part of the au- 

 ditory apparatus, the membrane of the tympanum, capable of 

 various degrees of tension, which is thrown into vibration in 

 obedience to waves of sound- conducted by the atmosphere ; 

 and it will be an important point to determine how far the 

 vibrations of this membrane are affected by the laws of the 

 production of tones by influence. 



After what w r e have learned of the laws of musical vibra- 

 tions, it will be easy to comprehend the production of tones 

 by influence. We shall take first the most simple example, 

 applied to strings. If we gently touch the note C upon the 

 piano, so as to raise the damper but not sound the string, and 

 then sing a note in unison, the string will return the sound, 

 by the influence of the sound-waves. The sound thus pro- 

 duced by the string will have its fundamental tone and over- 

 tones ; but the series of overtones will be complete ; for none 

 of the nodes are abolished, as in striking or plucking the 

 string at any particular point. If, instead of the note in 

 unison, we sing any of the octaves, the string will return the 

 note sung ; and the same is true of the 3d, 5th, etc. If we 

 now strike a chord in harmony with the undamped string, 

 this chord will be exactly returned by influence. In other 

 words, a string may be made to sound by influence, its funda- 

 mental note, its harmonics, and harmonious combinations. 

 To carry the observation still farther, the string will return, 

 not only a note of its exact pitch and its harmonics, but 

 notes of the quality of the primary tone. This is a very im- 

 portant point in its applications to the physiology of hearing, 

 and can be readily illustrated. Taking identical tones in suc- 



See page 184. 



