CHAP, iv.] HEARING. 1367 



and varied music ; at such times we seem to be the subject of a 

 " silence which, can be heard." 



847. As in the case of visual sensations, so likewise in the 

 case of auditory sensations the duration of the sensation is longer 

 than that of the action of the stimulus, the auditory sensation lasts 

 after the waves of sound have ceased to fall upon the ear. Hence 

 when two sensations follow each other within a sufficiently short 

 interval, they are fused into one. Since a membrane, thrown into 

 vibrations by a passing sound may continue to vibrate after the 

 sound has ceased, we might perhaps expect that this would be 

 the case with the tympanic membrane, and that hence the 

 interval of fusion would be longer in the case of hearing than in 

 that of vision, for in the latter case we have no corresponding 

 behaviour of any part of the dioptric apparatus. But we have 

 seen ( 816) that the acoustic arrangements of the tympanum 

 very rapidly damp the tympanic membrane ; and, as a matter of 

 fact, the interval in question is decidedly shorter in hearing than 

 in vision. Visual sensations separated by less than y 1 ^ sec. become 

 fused ( 749); but auditory sensations separated by not more 

 than yJ^ sec. may remain distinct ; if two seconds pendulums be 

 set swinging not quite in accord with each other and made to 

 tick, the tick of the one can be distinguished from that of the 

 other even when they differ in time by not more than -^ sec. 



848. When two notes are sounded at the same time the two 

 sound waves (we may suppose the notes to be pure ones, consisting 

 of a fundamental tone only without partial tones) do not travel as 

 two separate waves, but are compounded as we have already said, 

 into a single wave, the characters of which will depend on the rela- 

 tive characters of the two constituents. If the two notes have the 

 same period, that is to say are identical, the effect will be simply 

 an increase in amplitude ; the compound wave will have its crests 

 higher, and its troughs deeper than those of either of the single 

 waves, but will otherwise be like both of them. If two tuning- 

 forks of exactly the same pitch be struck, the sensation which we 

 experience is the same as that which we experience from either of 

 them alone, only more intense ; the sound is louder. 



If however the two tuning-forks are not of the same pitch, but 

 so related that the period of vibration of the one is not an exact 

 multiple of that of the other, the sensation which we experience 

 when the two sound together has certain marked features. We 

 hear a sound which is the effect on our ear of the compound wave 

 formed out of the two waves ; but the sound is not uniform in 

 intensity. As we listen the sound is heard now to grow louder 

 and then to grow fainter or even to die away, but soon to revive 

 again, and once more to fall away, thus rising and falling at regular 

 intervals, the rhythmic change being either from sound to actual 

 silence or from a louder sound to a fainter one. Such variations 

 of intensity are due to the fact that, owing to the difference of 



