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XXXVI. On Binaural Audition. By SiLYANUS P. Thomp- 

 son, B.Sc, B.A., Lecturer in B.iyerimental Physics^ Uni- 

 versity College^ BristoV^'. 



1. "T^URTXG the course of a series of experiments made a 

 -!->' few months ago I had occasion to listen to a tone of 

 slightly fluctuating pitch, the sound of which was led to one 

 ear by a caoutchouc tube. Desiring to measure the pitch of 

 the tone, it occurred to me to try to obtain interference-beats 

 with a fork of very nearly the same pitch. Under the cir- 

 cumstances this could only be accomplished by holding the 

 vibrating standard fork to the other ear. On doing this I 

 was somewhat surprised to observe that the "beats" were 

 heard with remarkable distinctness. Up to this time I was 

 not aware of any experiments on the binaural estimation of 

 beats, nor even that they could be so heard. Shortly after- 

 wards I learned from Dr. W. H. Stone that he has been in 

 the habit of using both ears, with a tuning-fork applied to 

 each, in counting beats, and that he finds no difference be- 

 tween the result of this method and that of listening to both 

 forks with one ear. 



2. When two forks producing ''beats" are thus applied, 

 one to each ear, there is a possibility of the vibrations of the 

 one fork being conveyed to the other ear exteriorly through 

 the air. To eliminate as nearly as possible this chance I took 

 two UT3 (c^ = 256) forks, by Ducretet of Paris, and loaded 

 one with wax to give about 2 '5 beats per second when 

 sounded with the other. The forks were unscrewed from their 

 resonant boxes ; an india-rubber tube was inserted into an 

 aperture in the wall of each of the resonant boxes correspond- 

 ing to the forks ; and the two tubes were led to the two ears 

 of the observer, and plugged round with cotton-wool. For 

 the sake of greater precision, the tubes were led away from 

 the apartment into two others, in each of which an assistant 

 struck one of the forks and held it in front of the mouth of 

 the resonant box. The "beats " were most distinctly heard, 

 and seemed to be taking place within the cerebellum. When 

 the forks vibrated so slightly as to be singly all but inaudible, 

 the beats of the two were most distinctly heard. 



3. When the beats were augmented in number per second 

 by additional loading, they were still very distinct to binaural 

 audition, and unusually harsh. On listening attentively for 

 a combinational (difference) tone, none was observed. Two 



* Communicated by tlie Author, liaying' been read Ijefore tlie British 

 Association; Section A, August IG, 1877. 



