322 Mr. T. J. Bowlker on the Factors serving 
difference of about half a wave-length in the arrival of the 
sound at the two ears — the image being on that side at which 
the sound-wave arrives first. 
It will be noted that in the earlier stages a displacement 
was more readily produced when the phase was advanced on 
the left ear. I was thus asymmetric with regard to my 
hearing, and I have found the same to be true of others. 
I also found that the hearing mechanism connected 
with the formation of the position of the image is capable of 
fatigue. After listening intently for some time with one ear 
towards the source, the position of the image does not return 
to exactly the same place as before. I tried to determine 
whether fatigue for one note resulted in a lessened displace- 
ment of the sound-image of another note, but 1 was unable to 
settle this point. These phenomena may be of interest to 
the experimental psychologist. 
Similar experiments were tried with notes of other wave- 
lengths, the result always indicating that there was a sudden 
appearance of an image on the other side of the field of 
sound- view when the difference of phase at the two ears was 
approximately half a wave-length. The maximum angle of dis- 
placement increased up to 90° as the wave-length of the sound 
increased ; a wave-length of about 36 inches being the first 
that gave an image displaced 90°. Sounds with longer wave- 
length gave a displacement of 90° before a phase-difference 
of half a wave-length was reached, and the sound-image then 
seemed to spread over a continually increasing length of arc 
on each side of 90°. 
It is hard to determine the actual position of a sound- 
image when the observer is ^cing_.thc source., and the sound- 
image is displaced through a large angle. Up to 20° I think 
I can tell the position to within 2°, from 20° to 30° to within 
4°, from 30° to 40° to within 6°, from 40° to 50° to within 
8°, after 55° I may be 10° or more out. When possible I 
find it best to point with the arm outstretched in the direction 
ifrom which the sound appears to come, but when holding 
,two tubes to the ears one cannot do this. 
In the above experiments resonance effects were still 
rather disturbing, and, as the general behaviour of the sound- 
images had been ascertained, a new arrangement was 
introduced. 
In this the ear-tubes were of equal length, and a right- 
angled bend was inserted in each, so that the outer portions 
of the tubes were approximately vertical. The plane through 
the outer edges of these tubes was horizontal. 
