450 Proceedings of Royal Society of Edinburgh. [sess. 
sensation in both experiments, viz., a localising of the sound in 
the ear. 
(2) Our power of estimating the direction of a sound is much 
more accurate for high notes and compound tones than for low 
ones. ISTow, it may be shown by a simple mathematical process 
that changes in the position of the source of a sound which 
reaches both ears will in general produce greater alterations in the 
phase with which it strikes the two ears when the note is high 
than when it is low. 
(3) Prof. Thompson has shown that the two ears do appre- 
ciate a difference in the nature of the sound when the phases 
are always exactly opposite in the two ears ; and it is not a very 
great step to suppose that our ears will perceive differences when 
one tympanum is repeatedly, though not always, moving in a 
direction opposite from the other. 
It must not be supposed that this perception of the difference 
phase by the two ears is necessarily an auditory sensation. 
Indeed, in some experiments its most prominent feature is not 
of that nature, e.g., if the sound of a tuning-fork is led to the 
two ears in such a way that the phases are exactly opposite, the 
sound is, in its auditory characters, pitch etc., the same as in 
ordinary hearing, but it seems to come from the middle of the 
head; thus the peculiar feature in this case is its locality, not 
its auditory character. 
Knowing, therefore, that by the aid of two ears we are enabled 
to perceive differences of phase, and having seen that this percep- 
tion is not necessarily auditory, it remains to consider by what 
faculty we do perceive the difference. 
In the middle ear there are two muscles, the tensor tympani 
and the stapedius, and they are inserted into the malleus and 
stapes respectively in such a way that with a movement inwards 
of the membrana tympani the tensor is slightly relaxed and the 
stapedius is rendered tense, and, conversely, outward movement 
of the membrane produces an opposite result in each muscle. 
By this means, therefore, it would appear that we have the 
faculty of appreciating the direction of movement of the tym- 
panic membrane occurring simultaneously in the two ears; or, to 
put it in other words, we may be able to perceive variations of 
