184 Prof. A. Steinhauser on the Theory 



minish their quantity, it will be necessary that the pinnae of 

 the ears should form tangent planes to the sides of the head. 

 This is in reality approximately the case. 



Further, to fulfil the condition that no sound coming from 

 the front shall be heard indirectly only, it is necessary that the 

 surfaces of the pinnae of the ears should not, when produced, 

 intersect in front of the apex of the nose, n (figure 1) ; for 

 in that case no sound-ray proceeding from the space included 

 between the produced surfaces and the face could enter either 

 ear directly. 



Every portion of the pinnae which reflects into the auditory 

 meatus a sound-ray that falls upon it may be called, with re- 

 spect to the direction of this sound-ray, an effective element of 

 the pinna. The sum of all the effective elements for sound- 

 rays in any given direction constitutes, for that direction, the 

 effective surface of the pinna. 



The effective surface of the pinna is consequently different 

 for every different direction of sound-rays. Also the intensity 

 with which the sound is perceived depends upon the magni- 

 tude of the effective surface. If then, as is found to be the 

 case, the intensity of the sensation of sound be not essen- 

 tially, or even disproportionately changed by a turn of the 

 head (which in effect is equivalent to a change in direction 

 of the sound-rays) , it follows that the magnitude of the effec- 

 tive surface must be nearly equal for all possible directions of 

 sound-rays. 



The pinna therefore fulfils a special service as a reflecting 

 curved surface ; and it might be possible, if time allowed of a 

 protracted and attentive study of the point, to discover the 

 particular purpose of each of its portions, and to discover its 

 normal form. 



In its essential construction the pinna of the human ear 

 consists of a funnel to collect the sound, and a reflector. The 

 former, a (figure 2), serves to take up the sound-rays that 

 come from the side, and behind ; the latter, b, to reflect those 

 rays which come from the front, and also from the side, this 

 reflexion being frequently accomplished with the aid of the 

 rim of the ear S, as figure 2 also shows. 



Before turning to the theory of binaural audition, the ques- 

 tion might be discussed why a man possesses two ears. For 

 this fact, the following reasons might be assigned : — 



First, the law of symmetry. This reason, however, is open 

 to exception, since a single organ of hearing situated on the 

 axis of symmetry of the body would not destroy its sym- 

 metry. 



Secondly. The beneficial provision of nature which endows 



