230 Lord Rayleigh on our 



modern adult heads to those of our ancestors, perhaps in a 

 very distant past. It is fortunate that when difference of 

 phase fails, difference of intensity comes to our aid. Perhaps 

 it is not to be expected that we should recognize intuitively 

 the very different foundations upon which our judgement rests 

 in the two cases. 



A rather difficult question arises as to whether in the labo- 

 ratory experiments it is possible to distinguish the phases of 

 agreement and of opposition. Not unnaturally perhaps, the 

 apparent movements of the sound from right to left and back 

 are liable to be interpreted as parts of a general movement 

 of revolution, so that, for example, phase-agreement may 

 correspond to the front and phase-opposition to the back 

 position. In a particular case the question is as to the 

 direction of the revolution, whether clockwise or counter- 

 clockwise. With respect to this, my observers frequently 

 disagreed, from which I am disposed to conclude that in these 

 experiments phase-agreement and opposition are not definitely 

 connected with front and back sensations. 



At this point there seems to be some discrepancy with the 

 observations of Prof. S. P. Thompson, who found* that 

 <c when two simple tones in unison reach the ears in opposite 

 phases ; the sensation of the sound is localized at the back of 

 the head." In Prof. Thompson's most striking experiment 

 a microphone is connected in series with a battery and two 

 similar Bell telephones, one of the telephones being provided 

 with a commutator by which the direction of the current 

 through it can be reversed. When the current flows similarly 

 through the telephones, a light tap near the microphone is 

 heard in the ears ; but when the current is reversed in one of 

 them a sensation is experienced " only to be described as of 

 some one tapping with a hammer on the back of the skull from 

 the inside" In some (rather inadequate) experiments I have 

 not succeeded in repeating this observation. 



The other branch of the subject, which I had hoped to 

 treat in this paper, is the discrimination between the front 

 and back position when a sound is observed in the open ; but 

 various obstacles have intervened to cause delay. Among 

 these is the fact that (at 64 years of age) my own hearing 

 has deteriorated. Thirty years ago it was only pure tones, or at 

 any rate musical notes free from accompanying noises, that 

 gave difficulty. Now, as I find to my surprise, I fail to 

 discriminate, even in the case of human speech. It is to be 



* Phil. Mag. November 187c8, p. 391. 



