418 
DR. W. S. TUCKER AND MR. E. T. PARIS ON 
at the same time the octave becomes more and more prominent. For some value of 
0 exceeding blit varying somewhat with different grids and the way in which the 
microphone is held, the fundamental is almost suppressed and the octave is heard with 
corresponding clearness*. When 6 is still further increased the fundamental becomes 
gradually restored, and the sound of the octave is altogether lost to the ear w'hen 6 = tt. 
The value of 6 at which the octave is most clearly heard (generally between 110° and 
120 °) is altered slightly if the microphone is rotated about its own axis. 
To demonstrate this effect, a microphone (M) was mounted with its neck projecting 
i--- 1 into a tube (TT) (fig. 14), which could be rotated 
AT about a horizontal axis. When so rotated the axis 
of the microphone could be inclined to the vertical at 
J m any desired angle, while the open ends of the tube (TT) 
14- were exposed to the same amount of sound throughout 
the experiment. The microphone was connected to an amplifier in the usual manner, 
and the output terminals of the amplifier were joined to a vibration galvanometer. 
The source of sound used was an electrically maintained tuning-fork making 250 vibra¬ 
tions per second, the microphone and vibration galvanometer being also tuned to this 
frequency. 
The curve in fig. 15 shows the deflection of the vibration galvanometer plotted 
