330 
Mr. T. J. Bowlker on the Factors serving 
nearer the source, the other vertical and further from the 
source. 
The position of the image was noted, and the tubes were 
then rotated through 180° about their axis, and the position 
of the image again noted. 
This arrangement made the conditions at both ears pre- 
cisely the same as regards resonance and intensity for both 
observations, the only difference being that there was a 
difference of 2*4 inches in the length of the path of sound to 
the two ears in the two cases. 
With this apparatus the results with two observers, myself 
and Mr. 0. B. Clarke, were as follows : — 
Wave-length 
of sound. 
Angular displacement of image. 
Observer O. B. C. 
Observer T. J. B. 
Inches. 
40 
18° 
18 
20 
18 
24 
18 
IS 
12° 
14 
14 
14 
16 
14 
12 
26-8 
194 
138 
10-4 
8-0 
6-4 
It is remarkable that the angle of displacement should 
remain nearly the same with such a wide variation in wave- 
length. 
Now if we take the diameter of the head through the 
ears to be 5J inches, a movement of the head of 25° 
would make a difference of 2 T \ inches in the distances of 
the ears from the source of sound. 
Assuming that when the head is turned through 25° the 
direction of the source is indicated exactly by the sensa- 
tion of the image, this experiment shows that in the case 
of one observer an average of 6°, and in the case of the 
other observer of 11°, had to be made up by the displace- 
ment of the sound-image due to intensity or other cause. 
As regards myself, I find that when my right ear is 
turned partly towards the source of sound and with eyes 
shut I point to it, T underestimate the angle from the 
symmetrical head-plane for sounds of 24 inches wave-length 
and over, and overestimate the angle when the left ear is 
turned towards the source. 
With regard to the general intensity of: the sound, I 
find that when the sound is fairly loud the apparent 
