386 Prof. S. P. Thompson on Binaural Audition. 



ploying a system of telephones, the two receiving-telephones 

 being reversed in action. 



For simple tones, then, the acoustic "image" does not vary 

 in position with pitch, if the difference of phase remains always 

 a maximum. For compound tones the first and second of 

 these methods of experiment are impossible ; but when using 

 the two telephones, I obtain the localization in the same posi- 

 tion for sounds of all pitches and kinds. Indeed the observa- 

 tion of articulate speech appearing to come from a spot at the 

 back of the head proves the phenomenon to be independent of 

 the pitch — that is to say, of the wave-length of the component 

 tones. 



I arranged a Hughes's microphone with two cells of Fuller's 

 battery and two Bell telephones, one of them having a com- 

 mutator under my control. Placing the telephones to my ears, 

 I requested my assistant to tap on the wooden support of the 

 microphone. The result was deafening. I felt as if simulta- 

 neous blows had been given to the tympana of my ears. But 

 on reversing the current through one telephone, I experienced 

 q, sensation only to be described as of some one tapping with 

 a hammer on the back of the skull from the inside. 



7. The relation of the acoustic " image " to the difference 

 of phase (b) between the two separate sounds is a more difficult 

 matter to deal with. The method of the telephone here be- 

 comes inapplicable, as it provides no means of partially chan- 

 ging the phase of one of the two sounds. The two sounds 

 must be either exactly coincident or exactly opposed in phase. 

 With simple tones, as of the tuning-fork, any desired differ- 

 ence of phase can be obtained by adjusting to the appropriate 

 amount the difference in length between the two conveying 

 tubes ; or by loading one of the two forks so as to make its 

 vibrations lose gradually on those of the other fork, and so 

 establish a regularly changing difference of phase. Both these 

 methods agree in showing, as far as accurate observation of 

 the subjective phenomenon is possible, that when the difference 

 of phase is partial, the sound is heard partly in the ears and 

 partly at the back of the head, the former partial sound dying 

 out and giving place to the latter as the difference of phase 

 approaches a maximum, and vice versa. 



A variation of this experiment may be of interest to record. 

 A stout copper wire about 3 feet long had its ends bent round 

 into smooth loops, and the whole was curved so that the two 

 loops could be inserted into the two auditory meatus of the 

 ears. An UT 3 (6 y = 256) tuning-fork was then struck; and its 

 stem was pressed against the middle point of the copper wire, 

 the vibrations having thus to travel equal lengths of the wire 



