Perception of Sound Direction, 'I'll 



does not arise when one sound in the open influences both 

 ears. 



Unless the open ends are pretty close to the ears, the sound 

 has more tendency to travel round the head to the wrong ear 

 than was observed at pitch 128. This may raise a question 

 whether, after all, the right and left effects may not be due 

 to small differences of intensities at the two ears varying 

 periodically. The best answer to this objection is to con- 

 sider what would be the consequence of such invasions. 

 Suppose that at one moment the vibration on the right is in 

 advance by one quarter period, so that a full right sensation 

 is being experienced. The retardation in travelling round the 

 head will at this pitch be about one quarter period, so that 

 the sound starting from the right in advance will on arrival 

 at the left be in approximate phase-agreement with the 

 principal sound there. On the other hand, the sound starting 

 from the left, already a quarter period in arrear, will on 

 arrival at the right be in approximate phase-opposition with 

 the principal sound on the right. The effect of travel round 

 the head is therefore to augment the sound on the left and to 

 diminish the sound on the right. This would evidently tend 

 to cause a sensation of sound on the left, and cannot therefore 

 be the explanation of the observed sensation of a sound on the 

 right. The same considerations will apply, if in less degree, 

 to sounds of pitch 128, and to sounds somewhat higher in 

 pitch than 256. 



The next sounds to be experimented upon in order of pitch 

 were from forks giving e' of 320 vibrations per second. 

 These could not be driven from the 128 per second inter- 

 rupters, and were merely held to the ears in the fingers of an 

 assistant. The right and left effect was very marked, but 

 there was a little difficulty at first in fixing the moment of 

 phase-opposition. After a few trials it became sufficiently 

 clear that the rule was the same as at the lower pitch, viz. 

 that the quicker fork asserts itself after the maximum of the 

 beat, corresponding to phase-agreement. 



From this point, as the pitch rises, the observations become 

 more difficult, partly no doubt on account of purely experi- 

 mental complications, but also, I believe, because the effects 

 are themselves less well marked. From two forks of pitch 

 </, electrically driven from the 128 interrupters and provided 

 with resonators in connexion with pipes, fairly distinct right 

 and left effects were obtained, but at first there were discre- 

 pancies as to which effect followed phase-opposition. These 

 appear to have been due to faulty observation of the phase 

 of opposition. As might have been anticipated, the moment 



