to determine the Direction of Sound. 327 
displacement * was very roughly proportional to the wave- 
length — for a wave-length of 4*4 inches the displacement 
would only be 5 C or G°. 
Now, to produce a difference of phase of half a wave-length 
the head, without the tubes, would have to be turned through 
about 23° — so if the head faced the source and was then 
turned to one side or the other, the image would appear to 
following the turning of the head if the intensity remained 
the same at the two ears, and when the head had turned 
through 23° the image of the source would have turned 
through 17° — this 17° must be compensated for by change 
of intensity at the two ears. As a matter of fact I am 
inclined to think that in the case of the higher notes — perhaps 
in the case of all notes — the zone or arc in which the sound- 
image appears is settled by the relative intensity at the two ears ; 
the actual position of the images within this zone being produced 
by the maxima and minima within it produced by phase- 
difference at the ears. 
In order to explain the existence of a movable image of the 
sound within this zone, we may suppose that the transmission 
of the sound impulse through some specialized part of the 
auditory apparatus or brain takes a definite time from each 
ear, and that the point where the impulses meet is the focus 
that gives rise to the sensation of a sound-image- 
To explain the existence of two images, and perhaps three, 
we may suppose « and /3 to be the crests of two successive 
waves ; then, if the observer is facing the source, the crests 
of a arriving at the ear simultaneously produce an image at 
the centre of the sound-zone ; a at the right ear and/3 at the 
left give an image to the right of the centre, and a. at the 
left ear and j3 at the right give an image to the left of the 
centre. 
If two equal tubes be applied to the ears, one with aperture 
2 inches in diameter, and the other of T 6 - of an inch, and both 
apertures be turned square on to the source, I find a 
deflexion of 20° to be produced in the sound-zone, and within 
this the image moves about as before. 
In order to determine whether intensity affected the 
position of the image when notes of medium pitch were 
concerned, one ear-tube was closed by a disk of aluminium 
luted on with clay. A hole was made through the centre of 
this disk, through which hole a cylinder of paper, 2 inches 
* Further experiments with wave-lengths down to 2 inches would, 
however, seem to indicate that this law does not hold even roughly for 
short wave-lengths, the displacement being much larger than this law 
would give. 
