344 Lord Rayleigh's Acoustical Observations. 



hearers described in my former paper. It was found that 

 Mr. Galton made mistakes which would be impossible for 

 normal ears, confusing the situation of voices and of clapping 

 of hands when to his right or left, as well as when in front or 

 behind him. Thus, when addressed loudly and at length by a 

 little boy standing a few yards in front of him, he was under 

 the impression that the voice was behind. In other cases, how- 

 ever, there seemed to be some clue, whose nature we could not 

 detect. Bad mistakes were made ; but the estimates were 

 more often right than mere chance would explain. 



After this experience it seemed unlikely that there could be 

 any success in distinguishing whether pure tones came from 

 right or left, and from in front or behind. The experiment 

 was tried, however, with in the main the expected result. 

 But when the sounds were close, there appeared to be some 

 slight power of distinguishing right and left, which may per- 

 haps have been due to incomplete deafness of the defective ear. 



A Telephone-experiment. 



In Maxwell's ' Electricity and Magnetism,' vol. ii. § 655, 

 it is shown that a perfectly conducting sheet acts as a barrier 

 to the magnetic force: — "If the sheet forms a closed or infi- 

 nite surface, no magnetic actions which may take place on one 

 side of the sheet will produce any magnetic effect on the other 

 side." In practice we cannot use a sheet of perfect conduc- 

 tivity ; but the above-described state of things may be approxi- 

 mated to in the case of periodic magnetic changes, if the 

 time-constants of the sheet circuits be large in comparison 

 with the periods of the changes. 



The experiment is made by connecting up into a primary 

 circuit a battery, a microphone-clock, and a coil of insulated 

 wire. The secondary circuit includes a parallel coil and a 

 telephone. Under these circumstances the hissing sound is 

 heard almost as well as if the telephone were inserted in the 

 primary circuit itself. But if a large and stout plate of copper 

 be interposed between the two coils, the sound is greatly en- 

 feebled. By a proper choice of battery and of the distance 

 between the coils, it is not difficult so to adjust the strength 

 that the sound is conspicuous in the one case and inaudible in 

 the other. 



Very high Notes. Rapid Fatigue of the Ear. 

 In former experiments with bird-calls I had often been 

 struck with what seemed to be the capricious behaviour of 

 these sources of sound, but had omitted to follow up the ob- 



