1004 AUDITORY SENSATIONS. [BOOK in. 



membrane, thrown into vibrations by a passing sound may con- 

 tinue 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 ( 614) 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 ^ sec. become fused ( 552); but auditory sensations 

 separated by not more than -j^ sec. may remain distinct; if 

 two seconds pend alums 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 y^ sec. 



627. 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 relative characters of the two constituents. 

 If the two notes have the same period, that is to say are iden- 

 tical, 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 other- 

 wise 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 a\vay. 

 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 pitch, the vibratory impulses of 

 the two sounds do not exactly correspond in time. Since the 

 vibration period, the time during which a particle is making an 

 excursion, moving a certain distance in one direction and then 

 returning, is shorter in one sound than in the other, it is obvious 

 that the vibrations belonging to one sound will so to speak get 



