8 PROCEEDINGS OP THE AMERICAN ACADEMY 



other, or to the passage of the vibrations through the skull from 

 the outer air. These experiments did not differ in other respects 

 from those of previous observers. The beating of the sound per- 

 sisting in one ear after excitation had ceased with a second sound 

 falling upon the other ear was, in fact, observed by S. P. Thompson, 

 in 1881. 



The existence of such binaural beats as are under consideration 

 was noted as early as 1874 by Mach, who assumed that the sound 

 was conducted from one ear to the other through the bones of the 

 head. S. P. Thompson has studied the subject very carefully;* 

 and while he is evidently strongly inclined to believe that the phe- 

 nomena are due to an interference of sensations not produced by the 

 same ear, nevertheless he does not appear to consider this as a fact 

 finally settled beyond all question, although to us his results seem 

 to leave small room for doubt on the matter. 



The important bearing of the existence of binaural beats upon 

 some parts of Helmholtz's theory of audition does not seem to have 

 been very fully recognized. This may be because of some question 

 as to whether each sound did not after all really act upon both ears, 

 even when apparently applied to one alone. From the nature of the 

 case, it is very difficult to prove finally that there is no possibility 

 of such conduction of sound from one ear to the other, but we have 

 sought to study the phenomena of binaural beats under circum- 

 stances which render such conduction improbable in the highest 

 degree. With this end in view, we have repeated most of the ex- 

 periments of Thompson, and made such additional tests as suggested 

 themselves to us. 



The sounds of two tuning-forks placed in a distant room were 

 conveyed to the ears telephonically over two separate circuits. 

 These forks were struck by hammers actuated by electro-magnets 

 placed in circuits separate from those containing the telephones, 

 and governed by keys operated by the foot of the observer at the 

 receiving end of the line. This method was found to be preferable 

 to running the forks continuously by electro-magnetic means, as 

 the currents used in the latter method were liable to act directly 

 upon the transmitting telephone and cause disturbing noises. Very 

 powerful magneto-telephones were used as transmitters and receivers. 

 Mii forks were used throughout the experiments, unless otherwise 



* Philosophical Magazine, Vol. IV. p. 274 ; Vol. VI. p. 383, Vol. XII. p. 351 ; 

 Vol. XIII. p. 406. 



