384 Prof. S. P. Thompson on Binaural Audition. 



sounds was demonstrated 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 

 resultant sensation, although the two sounds had had no op- 

 portunity 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 in- 

 tensity as to be singly inaudible. 



3. The apparent localization at the back of the head of 

 sounds whose vibrations reach the ears in opposite phases 

 being a subjective phenomenon, was announced by the author 

 as the result of the concurrent testimony of several indepen- 

 dent witnesses. Its existence was demonstrated by leading 

 separately to the two ears the sound of two unison tuning- 

 forks, one of which was slightly loaded to make its phase change 

 slowly relatively to the other. The "beats" were described 

 as being not " silences," as ordinarily observed when the dif- 

 ference of phase is half a complete vibration, yet as being 

 " most distinctly heard, and seeming to be taking place within 

 the cerebellum." Referring to this attempt to ascertain the 

 effect of bringing to the two ears waves of equal pitch and 

 intensity, but differing in phase, the author stated that a fuller 

 series of experiments was in course of completion. The sequel 

 of the present paper records with what result those experiments 

 have been made. 



4. The telephone of Graham Bell, introduced into this 

 country at the Meeting at Plymouth, where the former paper 

 was read, furnishes a new instrument peculiarly adapted for 

 researches of this nature. Both the phenomena recapitulated 

 above have been reobserved by a considerable number of inde- 

 pendent experimenters, confirming the results announced by 

 the author. Thus it was announced by Professor Graham Bell, 

 in a lecture before the Society of Arts, November 28, 1877*, 

 that these two phenomena had been discovered by Sir William 

 Thomson when experimenting at Glasgow with the telephone. 



One important advantage of the Bell telephone as an instru- 

 ment of research is that the phase of vibration can be inverted 

 at will, by reversing the connexions of the receiving-instru- 

 ment, so that the currents traverse the coil in a reversed direc- 

 tion, the motion of the vibrating disk being consequently also 

 executed in a reverse sense. Using two systems of telephones 

 to bring to the ears two sounds capable of yielding interference- 

 beats, interference in the resulting perception of the sounds is 



* Vide Journal of the Society of Arts, vol. xxvi. No. 1306, p. 22 

 (Nov. 30, 1877). 



