270 Prof. A. Steinhauser on the Theory 



From what has been said, it might readily be imagined that 

 a direct perception of sound can be discriminated from an in- 

 direct one more readily in the cases where the sound produced 

 by the sonorous body is a familiar one. 



Thus, for example, by the action of an echo, the familiar 

 voice of a friend, or the familiar roll of a waggon, with the 

 regular tramp of its horses, will suffer a certain indefinable 

 change scarcely consciously perceived, from which we shall 

 conclude, either from experience or conjecture, that the source 

 of sound is situated in the region of indirect binaural hearing, 

 and therefore behind us. 



This discrimination becomes far more difficult or even im- 

 possible if the source of sound and its varying tones be un- 

 familiar ; for if the sound be of a uniform character, such as 

 the tone of an organ-pipe, the buzzing of an insect, the various 

 elements of sound which by their united combination would 

 produce echoes occur at different times, and hence can exert 

 no such destructive effect on the character of the sound. But 

 discrimination is most difficult of all when the sound is of an 

 undecided or irregular nature, in which the separate elementary 

 sounds burst forth one after the other, as, for example, in 

 the sound of cattle-bells. 



As there are no other circumstances of any importance be- 

 sides those which have been adduced that aid in the percep- 

 tion of the direction in which a source of sound is situated, 

 and as these moreover afford no exact estimate of the direction, 

 we conclude that in the most favourable cases ice are only in the 

 position to decide that the source of sound is situated in the 

 region of indirect binaural hearing. 



If we would determine with any degree of exactness the 

 direction in which the source of sound is situated in such a 

 case, we must, as figure 11 shows, place before the ears/i and 

 f 2 , and parallel to them the planes h ± and h 2 ; for these planes 

 would reflect the rays arriving in the direction S so as to pro- 

 duce an impression which would coincide with that which 

 would be produced by a source of sound situated in the reversed 

 direction, S. 



9. As already previously remarked, direct sensations of 

 sound almost never occur without some admixture of indirect 

 sensations. 



It might therefore be contended that what has been said 

 about the indirect perception of sounds might also be equally 

 applied to their direct perception, and that consequently the 

 alleged distinction between the two series of perceptions was 

 no distinction at all. This objection may be met as follows : — 



Suppose two sounds produced simultaneously at two diffe- 



