92 
ON HEARING WITH TWO EARS. 
There are several ways of observing this singular subjective 
phenomenon. (1) The sound of a tuning-fork may he led to the 
two ears by two tubes whose lengths differ by half the wave- 
length of the sound employed, so that the vibrations reach the 
ear in opposite phases. (2) Two tubes, of equal length, leading 
separately to the ears may have their other ends placed opposite 
to two adjacent quadrants of the space surrounding a vibrating 
tuning-fork, in which case also there will be complete opposition 
of phase in the sounds that reach the ears. (3) While two tubes 
of equal length lead to the ears, two tuning-forks, tuned to 
perfect unison, are made to vibrate opposite the respective ends 
of these tubes, one of the forks being fixed while the other is 
slowly rotated around its axis. Here the sounds will arrive 
alternately in complete agreement and complete opposition of 
phase. (4) Lastly, we may transmit a sound electrically by any 
telephonic transmitter and receive it by the aid of a pair of Bell 
Telephones applied to the ears. If these telephones are joined 
up in the circuit, so that the current can be made to traverse 
the coils round their magnets, either in the same direction, or in 
opposite directions (the magnets being set similarly in each) the 
vibrations of the diaphragms will be either in agreement, or in 
opposition of phase. When they agree in phase the sounds 
appear to be localised in the ears, when they are opposed in 
phase they appear to be localised at the back of the head. 
In the case of some observers the sensation of any definite 
localisation fades rapidly away, to be revived again when the 
difference of phase of the two sounds is altered. 'Ihese phe- 
nomena of “ localisation ” have been observed many times by 
persons experimenting with the telephone, and they were inde- 
pendently announced (after the present writer had published 
them to the British Association) by Sir William Thomson and 
by Professor Graham Bell. With a microphone and a pair of 
receiving telephones the effect of localisation is remarkable. 
