ON HEARING WITH TWO EARS. 
95 
therefore, of finding the direction for the sound — say a lark 
singing high up in the air — will, according to Steinhauser, be as 
follows: — first the head is turned horizontally until the sound 
is equally loud in both ears, then the head is moved up and 
down until maximum loudness is attained ; when the lark will 
he found in the line of sight. Steinhauser’s theory, it will be 
noted, takes into account no differences of phase, pitch, or 
timbre, but of intensity only: and it fails to account for the 
fact that we have, ivithout moving the head at all, a very fair 
sense of the direction of sounds. 
13.— To test Steinhauser’s theory I have devised a little 
instrument called the Pseudophone, which is, for the ears, what 
Wheatstone’s Pendoscope was for the eyes — an instrument for 
verifying the laws of perception by means of the illusions which 
it produces. The Pseudophone consists merely of a pair of 
adjustable reflectors, or flaps, which can he fitted into the ears, 
and capable of turning round to any required position. By 
altering the position of the flaps we alter the relative intensities 
of two sounds as received in the ears ; and this can be done 
without the blindfolded experimenter knowing how the flaps 
are set. If, for example, the flaps are set to catch sounds from 
behind, the experimenter will think that he is looking in the 
direction of sound, when he is looking in precisely the 
reverse direction. But I find that the instrument fails to give 
satisfactory illusions with simple tones, such as those of tuning- 
forks, especially when the experiments are made out of doors. I 
cannot therefore accept Steinhauser’s theory without some 
considerable modifications. 
] 4. — Lord Kayleigh^ has pointed out that the diffraction of 
complex sounds round the head will produce the result that 
tones of different pitch arrive the opposite side of the head to 
^ Transactions of Musical Association, 1876. 
