220 Lord Rayleigh on our 



this evidence alone. And this conclusion leads to no difficulty; 

 for, as has already been explained, the difference of intensities 

 at the two ears gives adequate foundation for a judgement. 

 It would seem that at high pitch, above c", the judgement is 

 hased upon intensities; but that at low pitch, at any rate 

 below c (128), phase-differences must be appealed to. 



It remained to confirm, if possible, the suggestion that not 

 only are we capable of appreciating the phase-differences 

 with which sounds of equal intensity may reach the two ears, 

 but that such appreciation is the foundation of judgements 

 as to the direction of sounds — in particular of the right and 

 left effects. The obvious method is to conduct to the two 

 ears separately two pure tones, nearly but not quite in unison. 

 During the cycle, or beat, the phase-differences assume all 

 possible values ; and the mere recognition of the cycle is 

 evidence of some appreciation of phase-differences. Expe- 

 riments on these lines are not new. In 1877 Prof. 8. P. 

 Thompson * demonstrated " the existence of an interference 

 in the perception of sounds by leading separately to the 

 ears with india-rubber pipes the sounds of two tuning- 

 forks struck in separate apartments, and tuned so as to 

 'beat'' with one another — the ' beats ■ being very distinctly 

 marked in the resulting sensation, although the two sources 

 had had no opportunity of mingling externally, or of acting 

 jointly, on any portion of the air- columns along which 

 the sound travelled. The experiment succeeded even with 

 vibrations of so little intensity as to be singly inaudible." 

 And in an observation of my own f, where tones supposed to 

 be moderately pure were led to the ears with use of tele- 

 phones, a nearly identical conclusion was reached. But 

 although the cycle was recognized, in neither caseapparently 

 v\ as there any suggestion of a right and left effect. 



In repeating the experiment recently I was desirous of 

 avoiding the use of telephones or tubes in contact with the 

 ears, under which artificial conditions an instinctive judgement 

 would perhaps be disturbed. It seemed that it might suffice 

 to lead the sounds through tubes whose open ends were merely 

 in close proximity one to each ear, an arrangement which 

 has the advantage of allowing the relative intensities to be 

 controlled by a slight lateral displacement of the head towards 

 one or other source. Two forks of frequency 128, inde- 

 pendently electrically maintained, were placed in different 

 rooms. Associated with each was a suitably tuned resonator 



* Phil. Mag. Nov. 1878. 



f Phil. Mag. ii. p. 280 (1901) ; ' Scientific Papers/ iv. p. 553. 



