222 Lord Rayleigh on our 



although the best results require the position of the head to 

 be carefully chosen, right and left effects are perceived 

 through a considerable range. It is only necessary that the 

 intensities be approximately equal. 



These results are quite decisive, i£ we can assume that the 

 sounds were sufficiently isolated — that nothing appreciable 

 could pass from the open end of a pipe to the wrong ear. Ifc 

 was easy to verify that when one pipe and the opposite ear 

 were closed, next to nothing could be heard ; but it may 

 perhaps be argued that the test is not delicate enough. The 

 risk of error from this cause is diminished by approximating 

 the open ends to their respective ears. Many experiments of 

 this kind were made, but without influencing the results. 

 Finally, short lengths of rubber tubing were provided, by 

 means of which the ears could be connected almost air-tight 

 with the pipes. In this case, to avoid being deafened, it was 

 necessary to reduce the sounds by withdrawing the resonators 

 from their respective forks. The right and left effect re- 

 mained fully marked. Another argument to show that the 

 effects can not be explainediby sounds passing round the head 

 w r ill be mentioned presently. 



A question of great importance still remains to be considered. 

 Before the laboratory experiments can be accepted as expla- 

 natory of the discrimination of right and left when a single 

 sound is given in the open, it is necessary to show that in the 

 former the sensation o! right (say) is associated with a phase- 

 difference such that the vibration reaching the right ear leads. 

 There was no difficulty in obtaining a decision. While one 

 observer listens as described for right and left effects, a 

 second observes the maxima and minima of the beat as heard 

 by one ear situated symmetrically with respect to the two 

 sources. In the case of sounds of higher pitch to be con- 

 sidered later some precaution is required here; but for the 

 present sounds, corresponding to a wave-length of nearly 

 9 feet, there is no difficulty. Under good conditions the 

 minimum, represented by a silence, is extremely well marked, 

 and can often be signalled to within half a second. This, 

 signal, corresponding to opposition of phases, gives the required 

 information to the first observer. If a signal for the maximum, 

 representing phase-agreement, is desired, it is best made by 

 halving the intervals between the silences. 



The results can be stated without the slightest ambiguity. 

 The transitions between right and left effects correspond to 

 agreement and opposition of phase, not usually recognized 

 by the first observer as maxima and minima of sound. 

 When the vibration on the right is the quicker, the 



