ON HEAKING WITH TWO EARS. 
91 
perception of direction. A blind-folded man in a perfectly dark 
room is able to say, with very considerable accuracy, in what 
direction the source of any sound that may be made in the room 
is situated. In the open air he has a similar perception of the 
direction of a sound, but is liable to be deceived by sounds of a 
certain class. It has been my aim more particularly to bear 
this matter in mind during my investigations, which have a 
direct bearing upon it. 
The facts which I have brought to light are of the highest 
importance both for the physicist and the physiologist. I 
cannot claim, however, to rank either as a physiologist or an 
anatomist ; and the methods of research employed have been 
purely physical. I will give the results of my investigations as 
briefly as possible. 
3. — It is 2 ^ossible to produce an “ interference” in the perception 
of sounds. I prove the existence of this interference by the following 
simple experiment : — Let two tuning-forks, in unison with one 
another, be taken, and let one be loaded with a pellet of wax so 
as to vibrate a little more slowly than the other. When these 
two tuning-forks are excited by striking or bowing, and placed 
near one another so that their vibrations are communicated to 
adjacent masses of air, we have the phenomenon of interference 
commonly known as “ beats.” If, now, the sounds of these forks 
are separately led to the two ears by means of india-rubber tubes 
the beats are still heard ; and they appear to be taking place in 
the interior of the head. They can be distinguished even when 
each of the sounds is too feeble to be heard separately, and when 
every precaution is taken to guard against the actual com- 
mingling of the sound waves. They are even heard when the 
Eustachian tubes of the ears are closed during a catarrh. 
4. — If two simple sounds in unison ivith one another reach the 
ears in opposite phases^ the resulting sensation instead of being 
localized in the ears appears to proceed from the bach of the head. 
