to determine the Direction of Sound. 325 
Note of Wave-length 10*4 inches. 
Zero at 3° r. 
"Two-image point "... 13° L, 18° r., 41° 1. 
New images 24° L, 23° r., 50° 1. 
Cross-over angle 15° to 15°. 
Wave-length from first pair of two-image points 
= 2(31 sin 15i°) =16-7 inches. 
From first pair of new images 
= 31 sin 23i°= 12-4 inches. 
From next image 
31 sin 53° 1 . . , 
= - = 12*4 inches. 
Below this wave-length the images were too near together 
to have their position clearly determined, the sound of one 
image confusing the apparent position of the other image. 
Sometimes it appeared as if three images were in the field 
together, then the one nearest the source would appear the 
loudest and draw off the attention from, and apparently 
obscure the image directly in front of. the observer. 
The tubes were shortened so that the distance between the 
centres of their ends was reduced to 15 inches. 
With this pair of shortened tubes the following results 
were obtained : — 
Wave-length 10*4 inches. 
Zero at 0°. 
" Two-image point "... 22° 1. 
New images at 41° 1. & 40° r. 
Cross-over angle 20° 1. to 20° r. 
Here the two images were heard with the head facing any- 
where over the arc from 12° r. to 30° r. and from 12° l.to 30° 1. 
Wave-length by two-image point 
= 2x15 sin 22° = 11-1 inches. 
Wave-length by new images 
= 15sin40i°=9-8 inches. 
Wave-length 8 inches. 
Ci Two-image point " — two images were evident during 
nearly the whole range — there were practically always two 
and sometimes three images evident, though I had some 
doubts whether the central image was always real or a result 
of attention to the two side images. 
New images 24° 1. & 21° r. 
Cross-over angle 12°, i. e. the maximum angle of displace- 
ment was 12°. 
Wave-length by new images = 15 sin 22i = 5*7 inches, 
