of Binaural Audition . 271 



rent points, but such that their respective "timbres " or cha- 

 racters are somewhat different. We find by experience that 

 the single ear does not sum up the impressions received from 

 the two sources, but that the conceptions evoked by the im- 

 pressions received in the ears from the one source of sound are 

 separate from those evoked by the corresponding impressions 

 received from the other source of sound, so that the presence 

 of two separate sources of sound is known right well. 



This severing of the separate perceptions of sound becomes, 

 however, more difficult the more nearly the sources of sound 

 resemble one another. 



But that the ear is accustomed to such ah analysis of the 

 sensations, and that its analyzing-power may be whetted by 

 practice, is proved by the power acquired by the conductor of 

 an orchestra, who simultaneously follows the sounds of every 

 separate instrument. Again, imagine, as in figure 12, that 

 there is but one source of sound S, but that, in consequence of 

 reflexion at the wall W, indirect rays of sound also reach the 

 ears. Then the united effect of these direct and indirect rays 

 is obviously equivalent to that which would be produced by 

 two sources of sound, one situated at S, the other at Si- 



But the ear, by greater attentiveness and practice, resolves 

 these two sounds, which apparently proceed from tivo separate 

 sources, and which reach it in quick succession, and in resolving 

 them allows itself to be led to form a perception of the direction 

 by the direct effect alone, passing the indirect effect by unnoticed. 



If, on the contrary, this analysis is not achieved, by reason 

 of want of attention or of practice, or in consequence of defec- 

 tive knowledge as to the finer characteristics of the direct 

 sound (which an echo never completely repeats, and in con- 

 sequence of which singular changes in the tone of the echo 

 may be noticed), then acoustic illusions may occur, several 

 examples of which have been already mentioned in the pre- 

 ceding paragraphs. 



The ear, however, is no mere physical apparatus, but a sen- 

 sitive organ of mind capable of being trained. 



3. Theory of Mixed Binaural Audition 



10. If a source of sound is situated in the region of mixed 

 binaural hearing, then, as is known, the direct rays of sound 

 can reach only one of the two pinnae, while both may be reached 

 by the indirect rays. 



Accordingly let i x be the intensity with which the direct 

 rays of sound affect (say) the left ear, and p 1 the increment 

 of that intensity clue to the effect of reflexion. Let p 2 be the 

 intensity of the sensation in the right ear, and due to the re- 



