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XLIII. The Pseudophone. By Silvanus P. Thompson, B.A., 

 D.Sc, Professor of Experimental Physics in University Col- 

 lege, Bristol*. 



THE Pseudophone is an instrument for investigating the 

 laws of Binaural Audition by means of the illusions it 

 produces in the acoustic perception of space. It is therefore 

 the analogue for the ears of the Pseudoscope of Wheatstone, 

 which serves to illustrate the laws of BinocularVision by means 

 of the illusions it produces in the optical perceptions. 



The author has for some months been occupied with an ex- 

 perimental and theoretical investigation of the question of 

 binaural hearing, the chief points hitherto considered being 

 the subjective perceptions of two sounds led separately to the 

 ears, and differing in pitch, phase, or intensity. The results 

 of these investigations were communicated to the British 

 Association in the years 1877 and 1878, and have been pub- 

 lished in the Philosophical Magazine for 1877 and 1878. 



Independently of the work of the author, the theory of Bin- 

 aural Audition has been attacked by Prof. Anton Steinhauser 

 of Yiennaf , who has, however, treated the subject from a 

 somewhat different point of view, and has carefully developed, 

 by geometrical and algebraic reasoning, the laws of the rela- 

 tive intensities with which sound-waves reach the ears from 

 sources of sound situated at various points in front, or at the 

 side, or back, of the observer. In thus calculating accord- 

 ing to known geometrical laws the intensities of sounds which 

 reach the ears, Professor Steinhauser neglects such accessory 

 effects as might be produced by differences of pitch, or differ- 

 ences in phase, or diversify of quality of the sounds, and 

 assumes that these have nothing to do with the acoustic per- 

 ception of the direction in which a sound lies. He assumes 

 that that perception is based solely upon the relative intensi- 

 ties of the two sounds ; and upon this assumption his conclu- 

 sions are indisputable, being simply geometrical deductions 

 from the postulates of the problem. 



The author has, however, shown that differences of pitch 

 and of phase play a very important part in the subjective phe- 

 nomena of audition. His experiments with the simple tones 

 of tuning-forks, which were transmitted by mechanical or 

 electrical means to the ears in such a manner as to produce 

 required differences of phase, showed that difference of phase 



* Communicated by the Author, having been read before Section A of 

 the British Association at Sheffield, August 22, 1879. 



t Vide Steinhauser, " Theory of Binaural Audition," Phil. Mag\ April 

 and May 187S). 



