to determine the Direction of Sound. 323 
With this arrangement the resonance was the same in both 
tubes, and the intensity was also the same for both. 
The distance apart of the centres of the tubes was 31 inches, 
and their diameter was 2 inches. The tubes were luted into 
the right-angled bend with stiff modelling-elav. 
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"With this arrangement, when facing the source, the image 
did not always coincide exactly with the source, perhaps 
owing to the tubes being slightly unsvmmetrical with regard 
to the head when closely pressed against it to make a tight 
joint at the junction of head and tube. 
The method of procedure was — first to find the true zero 
by applying the ear-tubes and facing the source, then turning 
the head until the image appeared to be straight in front 
and then noting the reading. (The error here would pro- 
bably not be more than 1°.) Then the head was turned 
slowly to the left, this caused the image to move more or 
less to the right, at a certain position in the rotation of the 
head an image appeared suddenly to the left — this new image 
made an angle with the symmetric vertical head plane 
approximately equal to the angle the original image now 
made with it. This point is called the " two-image " point, 
and the angle that the image makes with the vertical head 
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plane at this point is called the " cross-over " angle. It is 
near the maximum displacement that phase will produce 
with the particular wave-length under observation. 
After this " two-image " point has been noted, the head is 
still turned towards the left until the new image appears 
directly in front. This point can usually be found within 
1° or 2°. The procedure is repeated to the right. 
The results obtained are as follows : — 
With ^sote of Wave-length 51 inches. 
Zero at 4° left. 
"Two-image" point at 57° left 3 no such point to the 
right. 
" Cross-over angle " from 90° right (wide image) to 90° 
left (wide image). These images were over several degrees 
of arc to right and left of 90°. 
The -two-image-" point 57° left, if we take it as occurring 
at a difference "of phase equal to half a wave-length, and 
reckon from the zero at 4r left, gives us 2 (31 sin 53°) = 49'6 
inches as the wave-length. 
