the two Ears in the Perception of Space. 415 



11. The writer therefore propounds the following theory, 

 which he believes to cover all the facts observed up to the 

 present time. 



Judgments as to the direction of sounds are based, in general, 

 upon the sensations of different intensity in the two ears ; but the 

 perceived difference of intensity upon which a judgment is based 

 is not usually the difference in intensity of the lowest or funda- 

 mental tone of the compound sound (or " clang "), but upon the 

 difference in intensity of the individual tone or tones of the clang 

 for which the intensity -difference has the greatest effective result 

 on the quality of the sound. 



It must be admitted that the acoustical perception of direc- 

 tion is, after all, not intuitive, but associative, just as the 

 optical perception of direction is. The ear has been trained 

 from childhood to associate certain differences in the quality 

 of sounds (arising from differences in the relative intensities 

 of some of the partial tones that may be present) with definite 

 directions ; and, relying on these associated experiences, 

 judgments are drawn concerning sensations of sound whose 

 direction is otherwise unknown. For sounds that are familiar 

 a difference of quality as heard in one ear will at once suggest 

 a direction. It is completely open to doubt whether a pure 

 simple tone heard in one ear could suggest any direction 

 at all. 



12. Acoustical Perception of Distance. — There remains for 

 discussion the acoustical perception of distance. 



In the case of known sounds we doubtless judge chiefly of 

 their distance by their relative loudness, the intensity decrea- 

 sing inversely as the square of the distance. The analogous 

 fact in optics is the perception of distance by the apparent 

 magnitude of objects familiar to the sight and of known 

 size. 



It might be possible indirectly, in the case of short dis- 

 tances, to judge of distance by "acoustic parallax" — that is to 

 say, by the difference in the direction of the sound as perceived 

 in the two ears. The quality of a compound sound also differs 

 slightly with distance (independently of direction) ; and from 

 this fact a judgment might be drawn. It must be held ex- 

 tremely doubtful, however, whether any such judgments are 

 actually made. 



13. Conclusion. — The foregoing arguments and deductions 

 have been based upon observations made, so far as the author 

 is aware, exclusively by and upon persons possessed of normal 

 powers of hearing. Nevertheless the subject is of such a 

 nature that its more obscure points would doubtless be greatly 



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