Perception of Sound Direction. 223 



sensation of right follows agreement of phase, and (what 

 is better observed) the sensation of left follows opposition of 

 phase. And similarly when the vibration on the left is the 

 quicker, the left sensation follows agreement, and the right 

 follows opposition of phase. The question which fork is 

 vibrating the quicker is determined in the usual way by 

 observing the effect upon the period of the beat of the addition 

 or removal of a small load of wax. If for example the beat 

 is slowed by loading the right fork, we may be sure that that 

 fork was originally the quicker. A large number of com- 

 parisons of this kind have been made at various times, and in 

 no case (at this pitch) has the rule been violated. It is not a 

 little remarkable that by merely listening to right and left 

 effects aided by signals giving the moment of phase-opposition, 

 it is possible conversely to pronounce wdiich fork is the 

 quicker, although the difference of frequencies may not exceed 

 *02 vibration per second. 



It should not pass unnoticed that the laboratory experiments 

 cover a wider field than the observations in the open. In the 

 latter case, if the single sound of pitch 128 is in front or 

 behind, there is agreement of phases at the two ears. As 

 the position becomes more and more oblique, the phase- 

 difference increases ; but it can never exceed a moderate 

 amount, about one-eighth of a period, which is attained when 

 the position is precisely to the left or right. Phase-differences 

 in the neighbourhood of half a period do not occur. From 

 the laboratory experiments it appears that the right and left 

 effects are not subject to this limitation ; but that, for example, 

 a right effect is experienced when the vibration reaching the 

 right ear leads, whether the amount of the lead be small or 

 whether it approaches the half period. 



The right and left or, as I shall sometimes say for brevity, 

 the lateral, sensations observable in this way are so conspicuous 

 that I was curious to inquire how I had cont lived to miss 

 them in the earlier experiments with telephones already 

 alluded to. The apparatus was the same as before. In the 

 neighbourhood of the electro-magnet chiving each fork (128) 

 was placed a small coil of insulated wire whose circuit was 

 completed through a telephone. The double wires connected 

 to the telephones were passed through the perforated wall. 

 In order to weaken the higher ovei tones, thick sheets of 

 copper intervened between the electro-magnets and the coils. 

 But when the telephones were held to the ears, the sounds 

 were perceived to be of a more mixed character than I had 

 expected ; and I am forced to the conclusion that I must 

 formerly have overestimated their approximation to the 



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