266 Prof. A. Steinhauser on the Theory 



ing the effects of reflexion on direct binaural hearing may 

 possibly not entirely coincide with the phenomena actually 

 exhibited, since the very conditions assumed as fundamental 

 in the argument are such as cannot in actual practice occur 

 all at once. Thus the rays of sound which fall upon the sur- 

 faces of the two pinnse and upon the reflecting wall were taken 

 as parallel to one another. Results obtained upon this as- 

 sumption are consequently only approximately correct when 

 the source of sound is remote from the hearer, and when the 

 surface (now not necessarily considered "large ") of the wall 

 which reflects the rays of sound to the ears lies very near to 

 the hearer, since only under these circumstances can the rays 

 which strike the pinnae and also the surface of the wall be re- 

 garded as at all nearly parallel. 



Lastly, the reflecting walls were assumed to be completely 

 reflecting; that is to say, it was assumed that the rays falling 

 collectively upon the surface of the wall were sent off in 

 straight lines parallel to one another. Since this is in actual 

 experiment untrue, for the rays of sound suffer a partial dif- 

 fusion, as do the rays of light at an imperfectly polished sur- 

 face, this circumstance implies a loss of the scattered rays, 

 which may be greater or less according to the nature of the 

 reflecting surface. 



7. In concluding the subject of direct binaural hearing, we 

 have only to add a few words on the perception of the change of 

 place of the source of sound by the sensations of sound which 

 are experienced when the head is not moved. 



Let us imagine a source of sound which completes a motion 

 within the region in which direct binaural hearing can occur, 

 and let us inquire whether, and in what way, the nature of the 

 motion can be learned from the sensation of sound. We shall, 

 on consideration, easily find the answer to this question on 

 referring to the theory developed in the preceding pages, as 

 follows : — 



The change of place of the source of sound has two conse- 

 quences : — 



(1) A change of the direction in which the rays of sound 

 reach the ears, and therefore a change in the ratio between 

 the intensities i x and i 2 with which the sound is perceived in 

 the two ears. 



(2) A change in the total intensity (ii + 2 ) "vvith. which the 

 the sound is perceived. 



The latter effect may result partly from a possible change of 

 the distance of the source of sound from the observer, and 

 partly from the change of the direction of the rays of sound. 



Disregarding the latter circumstance — for, as will presently 



