% .]26 Mr. T. J. Bowlker on the Factor* serving 
So far the tubes had been used with the full aperture — a 
circle of 2 inches diameter. Now they were closed by disks 
luted on with modelling-clay — the disks being perforated by 
holes -j^y of an inch in diameter. 
Xote of Wave-length 6*4 inches. 
Zero 4° 1. 
Images at 24° 1. and 22° r. ; but now three images could 
always apparently be heard together when facing one of 
them. When blowing the note hard the central image could 
be heard most strongly, but as the sound gradually ceased 
the central image died away, first leaving the impression that 
the two side images ouly existed. 
Cross-over angle — or maximum angle of displacement of 
image about 9°. 
Wave-length by new images =15 sin 23° = 5*8 inches. 
Xote of Wave-length 5*8 inches. 
Zero at 0°. 
Images at 16° 1. and 14° r. 
When facing the image at lti 1. the image further to the 
left seemed stronger and tended to draw off one's attention. 
When facing 14° r. the image at the source, which now 
appears only about 8° to the left, tends to obscure the image 
in front at 14° r. 
Maximum displacement 6° or 8°. 
Wave-length by images =15 sin 15° = 3*9 inches. 
Xote of Wave-length 4*4 inches. 
Here it was very difficult to determine even approximately 
the position of an image, the one to the left of the two or 
three in the field of view seeming the loudest as a rule. 
I thought I had images in front, however, at 8° r., 10° 1., 
22° L, 36° 1., but it was impossible to distinguish between 
having an image directly in front and having a two-image 
point in front. I only felt sure that phase was still playing 
a part in fixing the maxima and minima which gave rise to 
the centres of the sound images. 
From the measurements of the maximum angle of dis- 
placement that I had taken in the above experiments it would 
seem that for wave-lengths below 20 inches the angle of 
