CHAP, iv.] HEARING. 1373 



each corresponding to a vibration of a particular period, but as 

 a complex wave in which the simple waves are compounded 

 into one ; and the vibrations of the tympanic membrane, followed 

 by the vibrations of the perilymph, have the same composite 

 character. When for instance a note is sung, or sounded on 

 a musical instrument, the air in the external auditory passage 

 is not the subject of one set of waves corresponding to the funda- 

 mental tone, and of other sets corresponding to the several partial 

 tones, but vibrates in the pattern of one composite wave ; the 

 tympanic membrane executes one complex vibration, and a corre- 

 sponding single complex vibration excites the auditory epithelium. 

 And this holds good not for a single sound only but for a mixture 

 of sounds. We can in a clumsy way take a graphic record of 

 the vibrations of a dead tympanic membrane, by attaching a 

 marker to the stapes ; could we take an adequate record of the 

 movements of the living tympanum of one of the audience at a 

 concert, we should obtain a curve, a phonogram, which though a 

 single curve only would be on the one hand a record of the multi- 

 tudinous vibrations of the concert, and on the other hand a picture 

 of the actual blows with which the perilymph had struck the 

 auditory epithelium. 



Now, whatever be the exact nature of the process by which the 

 vibrations of the perilymph give rise to auditory impulses, we may 

 consider it as probable that, in giving rise to those impulses, 

 the complex vibration is analysed again into its constituent simple 

 vibrations, that the vibrations start afresh so to speak in the audi- 

 tory epithelium, marshalled in the same array as that in which they 

 started from the sounding instruments, as if the auditory epithelium 

 itself constituted the band playing the music. And indeed that 

 something of this kind does take place is indicated by the fact that 

 an adequately sensitive ear can in a musical sound detect one or 

 more of the partial tones as distinct from the fundamental tone, or 

 still more easily can in a mixed concert detect the several notes of 

 the several instruments, though as we have just said in the move- 

 ments of the tympanic membrane, all the constituent factors are 

 merged into one complex sweep. We may conclude then that we 

 possess some means of analysing the composite waves of sound 

 which sweep through the perilymph, and of sorting out their 

 constituent vibrations. 



There is at hand a simple and easy physical method of analysing 

 composite sounds. If a person standing before an open pianoforte, 

 the loud pedal being held down, sings out any note, it will be 

 observed that a number of the strings of the pianoforte will be 

 thrown into vibration, and on examination it will be found that 

 those strings which are thus set going correspond in pitch to the 

 fundamental tone and to the several partial tones of the note sung. 

 The note sung reaches the strings as a complex wave, but the 

 strings are able to analyse the wave into its constituent vibrations, 



