Prof. S. P. Thompson on Binaural Audition. 387 



before reaching the ears. The sound appeared to comt3 from 

 the ends of the wire in the ears. The stem of the fork was 

 now slid along the wire. At about an inch and a third from 

 its former position the difference of path of the two sounds 

 had been sufficient to produce complete difference of phase, 

 and the sound appeared to come, not from the two ends of the 

 wire, but from the back of the head. In intermediate positions 

 the effect was of a mixed character : part of the sound was 

 heard as in the ears, part at the back of the head. With 

 forks of various pitches, a similar result was found, with ap- 

 propriate differences in the lengths of path. 



8. The next experiments concern the relative intensities (c) 

 of the two sounds. When two simple tones in unison and 

 agreeing in phase are led to the ears, one tone being louder 

 than the other, the simple result of experience is that the sound 

 is heard in one ear more than in the other. It is as if the 

 sounds were loud, but as if one ear were partly deaf. But if 

 the two simple sounds differ by exactly half a vibration, and 

 one is louder than the other, the result is wholly different. 

 The sound no longer seems to be in the ears. There is an 

 acoustic " image " localized at the back of the head. Instead, 

 however, of this image appearing to exist at the centre of the 

 back of the head as it does when the sounds are equal in in- 

 tensity, it appears to be on one side, nearer the ear in which 

 the sound is louder. If two unison tones reach the two ears 

 separately, with complete difference of phase and equal inten- 

 sity, and then the intensity of one sound be gradually reduced 

 down to nothing, the acoustic image, which at first occupies 

 a position behind the middle of the top of the cerebellum, gra- 

 dually moves round the back of the head apparently just 

 within the skull, to the ear in which the sound arrives with 

 full intensity. I have verified this result with tuning-forks 

 and tubes, and also with the microphone and two Bell tele- 

 phones, in one of which the intensity of the vibration could 

 be regulated by adjusting the position of the magnet. 



9. The effect of the quality \d) of a compound sound on the 

 localization of an acoustic image in binaural audition is very 

 complicated. The action of the telephone in reversing the 

 phase of all vibrations independent of their quality has already 

 been noted in § 6. 



A case, however, occurred, of some independent interest. 

 Suppose a simple tone and its octave to be combined together, 

 so yielding a compound wave of definite form. Suppose now 

 the octave note to be simply reversed, and in this condition 

 compounded with the original fundamental tone. The form 

 of the second compound wave will exactly resemble that of 



2C2 



