of Bin aural Audition, 183 



It might conceivably appear that direct audition could only 

 occur under conditions of a special character, and therefore 

 extremely rarely, since the hearer is almost always surrounded 

 by reflecting surfaces, amongst which, of course, the earth 

 must be reckoned. 



Figure 1 exhibits the aspect of the human head from above, 

 /i and/ 2 being the surfaces of the pinnae. They make with 

 one another an angle 2/3. Hence it is easily perceived that 

 according to the position which the source of sound occupies 

 in the horizon of the hearer, the following cases may be dis- 

 tinguished with respect to binaural audition. 



1. The source of sound may be situated within the angle 

 DnC. The hearer then hears directly with both ears ; and ac- 

 cordingly there exists a direct binaural audition extending 

 through the angle DnC, which is equal to 2/3. 



2. The source of sound may be situated within one of the 

 angles AnD and BnC. The hearer then hears directly with 

 one ear only, whilst the other ear hears only indirectly in almost 

 every case, on account of a scattering of the sound, analogous 

 to the scattering of rays of light. In consequence a mixed binau- 

 ral audition occurs, and extends, irrespective of the region in 

 which both ears hear directly, through the angle (A n D + B n C), 

 which is equal to (360—4/3). 



3. The source of sound may be situated within the angle 

 AwB. The hearer in that case evidently hears only indi- 

 rectly with both ears ; accordingly indirect binaural audition 

 occurs, and extends through the angle AnB, which is equal to 

 2/3. 



The 360 degrees of the entire circle within w T hich the source 

 of sound is situated are therefore disposed as follows : — 



2/3 degrees in a region of direct binaural audition ; 

 360-4/3 „ „ mixed „ „ 



and 2/3 „ „ indirect „ „ 



giving the necessary sum of 360 degrees. 



The angle /3 differs naturally in different individuals, and 

 has been diminished particularly in the female sex by the 

 effect of the head-dresses worn for a long period. That this 

 operates prejudicially on the hearing, at least in so far as it 

 diminishes the extent of the direct binaural audition, may be 

 inferred from what has already been said. It will be in the 

 sequel more narrowly examined. It is self-evident that the 

 range both of the direct and of the indirect monaural audition 

 extends to 180 degrees. 



If the front part of the head is to offer to the sound-rays on 

 their way to the ears no obstacle of such a nature as to di- 



P2 



