of Binaural Audition. 187 



source, as in the preceding examples of the wagon, the chain, 

 &c, will therefore be looked out for by the eyes alone only so 

 long as the power of drawing a judgment as to the direction 

 is lacking to the ear. From the impressions which the sound 

 coming in a direction now known makes upon the ears, the ears 

 learn at last to recognize the direction in which the source of 

 sound is situated, without needing the further assistance of 

 the eyes. Thus every one always looks for the source of sound 

 straight in front in the line of sight whenever the impressions 

 on both ears possess equal intensity. A coincident result 

 may be deduced from equation (2), in which if ii=i 2 we have 

 tan a = 0, and hence a = 0. 



If, however, the effective surface of one of the two pinnse be 

 artificially enlarged by holding the hollow of the hand behind the 

 ear, and the intensity with which that ear perceives the sound 

 be consequently increased, the source of sound, even if it does 

 not change its real position, appears to move back toward 

 that side on which the pinnse has thus been enlarged. 



Suppose, for example, that, where originally ii = i 2 , by the 

 enlargement of the effective surface of the left pinnae %i be- 

 comes greater than i 2 . Then in equation (2) the values of the 



(proper) fraction } ? , and therefore also the value of tan a 



and of the angle a, become greater than they were before ; 

 and the direction which is conjecturally assigned to the source 

 of sound is moved on toward the left. 



Also by plugging one of the two ears more or less thoroughly 

 with cotton-wool, an apparent change of direction in the source 

 of sound can be brought about. 



In all these investigations the eyes must be kept closed, in 

 order that the judgment may not prejudice the illusion. A 

 point of importance in complete coincidence with the fore- 

 going observations is the phenomenon that individuals who do 

 not hear equally well with both ears, always look for the source 

 of sound more toward that side on which the ear of better 

 hearing is situated. Thus if the worse-hearing ear is the left 

 one, and it perceives a sound of intensity i ± with only the mth. 

 part of that intensity, then 



1 2 



tana =*=-! .tan /3 



m 



%i + mi 2 



