#ii Jearitrg Ufooi €ars. 
By SILVANUS P, THOMPSON, D.Sc„ B.A., RR.A.S. 
( Professor of Experimental Physics in University College^ Bristol.) 
J.—l^/rAN is provided with two ears, as well as two eyes. If 
-LfX he had not two eyes, it would be almost impossible for 
him to distinguish the distances and solid forms of objects ; for, 
as Wheatstone showed, the perceptions of solidity and of distance 
acquired through the eye are due to the fact of our haviug two 
eyes ; the former of these perceptions having for its starting 
point the slight differences between the two retinal pictures in 
the two eyes, the latter being based upon the muscular sensations 
of the greater or less convergence of the optical axes when 
viewing near or distant objects, or, as it is sometimes termed, 
upon binocular parallax. The theory of Binocular Vision was 
practically complete when the invention of the stereoscope, and 
of its reductio-ad-absurdum, the Pseudoscope, proved the correct- 
ness of Wheatstone’s theoretical views. 
Man has two ears ; and 'whatever view we take of the 
process of creation, theological or evolutionist, we must admit 
that the two ears, like the two eyes, serve a purpose which one 
ear could not serve alone. It has therefore been my endeavour, 
in a research carried on at intervals during several years, to 
investigate the functions of the two ears, in the hope of throwing 
light upon some of the unexplained facts in the perception of 
sound. These researches in Binaural Audition are therefore 
analogous in aim to those of Wheatstone in Binocular Vision. 
2. — ^One of the facts in the perception of sound which has 
never been satisfactorily explained is that of the acoustical 
