of Binaural Audition. 197 



or by what we know from practical experience, that in the case 

 of hearing a sound coining from above or below, we are able to 

 estimate, from the relative intensities with which the sound is 

 perceived in the two ears, the azimuth of the rays of sound as 

 projected upon the plane of best hearing. 



This explains the method which we pursue to discover the 

 position of a source of sound whose situation is unknown, and 

 which consists in a motion of the head. 



For example, to find a source of sound situated anywhere 

 above, we have only to turn the head about a vertical axis until 

 we hear equally with both ears and with the greatest total in- 

 tensity. Then the line of sight coincides with the horizontal 

 projection of the direction of the sound. The head must then 

 be turned upwards about a horizontal axis at right angles to 

 the line of sight, as long as the intensity of the sensation 

 increases. 



When this reaches its maximum, and is equally great for 

 both ears, then the source of sound must be situated in the 

 line of sight. 



And here we must remark on the essential importance of 

 the actual conditions — that the ears and eyes, being alike at- 

 tached to the head, share its movements, and that they are 

 situated at almost the same height above the ground. 



Should we perceive a falling-off in the intensity of the 

 sound on raising the head, this would be an indication that 

 the source of sound is situated below the plane of best hearing, 

 and that we should be able by sinking the head to bring the 

 source of sound into the line of sight in the manner just de- 

 scribed for a sound above the head. 



It is intelligible and natural that, where (as in an instrument 

 for measuring altitude and azimuth) we have two separate 

 motions of rotation at right angles to one another, it is imma- 

 terial whether the movements in the two directions be executed 

 separately or by a simultaneous motion of the head. 



This occurs, for example, when we try to find a lark which 

 we may hear singing above a field. We raise the head, 

 making an arbitrary guess at the position of the lark in the 

 sky. Then we turn the head about, led meanwhile by ear 

 until we hear equally well with the two ears and with the 

 greatest possible intensity ; and simultaneously we perceive 

 the lark in the line of sight. We do not, therefore^ as it might 

 be conjectured in this case, seek for the source of sound by 

 means of the eyes, but by means of the ears. 

 [To be continued.] 



Phil. Mag. S. 5. Yol. 7. No. 42. March 1879. Q 



