of Binaural Audition. 273 



by it, are only possible so long as the source of sound remains 

 in the region of direct binaural hearing. 



If, on the other hand, the source of sound is situated either 

 in the region of indirect or in that of mixed binaural hearing, 

 we may arrive at an exact knowledge of the direction in which 

 it is situated by turning the head while we seek to bring the 

 source of sound : — 



(1) Into the region of direct hearing, and preferably into 

 the direction of best binaural hearing. 



(2) Onto the boundary of two neighbouring regions of 

 hearing. Since the direction of this boundary is known to 

 each individual, the direction of the source of sound when 

 brought into coincidence with this direction is also known. 

 This auxiliary means offers in many cases the advantage that 

 it only requires a slight movement of the head. If, for ex- 

 ample, the source of sound be in the region of indirect hear- 

 ing, a very slight movement of the head suffices to bring it 

 onto the boundary between the regions of indirect and of 

 mixed hearing. 



(3) Into the known direction of best monaural hearing. 

 This happens when in doubtful cases we wish to decide 

 whether the source of sound is situated in front or behind. 

 For if we turn the head about a vertical axis, according as the 

 source of sound is situated before or behind, the intensity of 

 the sound will increase in the ear that is turned forward or 

 backward respectively, since by the movement of the head the 

 source of sound is brought more nearly or even quite into the 

 direction of best hearing for one of the two ears. 



As may also be deduced from the preceding theory, the 

 angle /3 which the surfaces of the pinnae make with the line of 

 sight, may, in certain circumstances, exercise no inconsiderable 

 influence upon binaural hearing. Since this angle is not 

 always alike in different individuals, but may differ within 

 certain tolerably wide limits, the consequences which the large- 

 ness or smallness of that angle may involve, are here summa- 

 rily enumerated. 



For, in accordance with the preceding theory, the larger 

 the angle /3 is, 



(1) the wider will be the region of direct binaural hearing, 



(2) the smaller will be the region of mixed binaural hearing, 



(3) the better and more distinct will hearing be in the line 

 of sight, 



(4) the larger will be the region of indirect binaural hearing, 



(5) the less certain will be the estimation of direction in the 

 region of direct binaural hearing, and, lastly, 



(6) the greater will be the possible difference in the total 

 Phil Mag. S. 5. Vol. 7. No. 43. April 1879. Y 



