834 SPECIAL SENSES. 



the 1st and 5th, the 1st and 3d, and other harmonious combinations, is so great that we 

 have no beats and no discord, the more rapid waves reenforcing the harmonics of the 

 primary sound. It is important to remember, in this connection, that resultant tones are 

 equal to the difference in the rates of vibration of two harmonious tones. If we take a 

 note of 240 vibrations, and its 5th, with 360 vibrations, these two have a difference of 

 120, which is the lower octave of the 1st and is a harmonious tone. 



It is evident that the laws which we have thus stated are equally applicable to 

 overtones, resultant tones, and additional tones, which have their beats and dissonances, 

 as well as the primary tones. 



Tones ~by Influence (Comonince). The term consonance is generally applied to the 

 harmonious combinations of two or more sounds, and is synonymous with accord, as it 

 is used in music. In this sense, it is opposed to dissonance, or discord. By some writers, 

 however, consonance is used to denote sounds produced in sonorous bodies by the influ- 

 ence of sounds in unison. If, for example, we have a bell tuned to a certain note and 

 bring near its opening a tuning-fork vibrating in unison with this note, the bell will 

 sound vigorously in unison, although it is influenced only by the vibrations in the air pro- 

 duced by the primary sound. We have already spoken of this under the head of reso- 

 nance ; and sounds produced in this way are properly called tones by influence. Some 

 physicists designate these as sympathetic vibrations. Dr. Elsberg, of New York, uses 

 the term co-vibration and co-sounding, as applied to these phenomena. 



It is evident that the mechanism of the production of tones by influence cannot be 

 neglected in studying the physiology of audition. We have, as an important part of the 

 auditory apparatus, the membrane of the tympanum, capable of various degrees of ten- 

 sion, 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 we have learned of the laws of musical vibrations, it will be easy to com- 

 prehend the production of sounds by influence. We shall take first the most simple 

 example, applied to strings. If we gently touch the note 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 produced by 

 the string will have its fundamental tone and overtones ; 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 

 fundamental tone, its harmonics, and harmonious combinations. To carry the observa- 

 tion still farther, the string will return, not only a note of its exact pitch and its harmon- 

 ics, but notes of the quality of the primary note. This is a very important point in its 

 applications to the physiology of hearing and can be readily illustrated. Taking iden- 

 tical notes in succession, produced by the voice, trumpet, violin, clarinet, or other musi- 

 cal instruments^ it can be easily noted that the quality of the note, as well as the pitch, 

 is rendered by a resounding string ; and the same is true of combinations of notes. 



The above laws of tones by influence have been illustrated by strings merely for the 

 sake of simplicity ; but they have a more or less perfect application to all bodies capable 

 of producing musical tones, except that some are thrown into vibration with more diffi- 

 culty than others. An interesting application of these laws, however, particularly with 

 reference to the physiology of the ear, is in the case of stretched membranes; for this 

 brings to our mind the possible action of the membrana tympani. 



If we have a thin membrane, like a piece of bladder or thin rubber, stretched over a 

 circular orifice, such as the mouth of a wide bottle, this can be tuned to a certain note. 



