94 
ON HEAEING WITH TWO EAES. 
10. — When two sounds constituting a simple interval are led 
separately to the ears, there is no differential resultant-tone 
(“ Tartini’s tone ”) ; at least I have never been able to hear one. 
I am disposed to attribute the unexpected harshness of the 
ordinary intervals to the absence of this differential tone, which 
would harmonise the two. It must, however, be added, that if 
the two tuning-forks used have only a narrow interval, and are 
capable of producing a clearly distinguishable differential tone, 
and if they are placed together near a Hughes’ Microphone to 
which two telephones are connected so as to give opposed phases 
of vibration, then not only will the two primary sounds be heard, 
but also their differential tone ; and they will all be apparently 
localised at the back of the head. 
11. — When a sound reaches an observer from a point right 
in front of, or behind, or above him, the length of path 
travelled by the sounds to the two ears is the same for all 
sounds. But if the source of sound be to right or left of the 
observer, the sound will reach the one ear a little later than 
the other, and with a difference of phase depending both on the 
wave-length of the sound and on the cosine of the angle between 
the line of the sound and the median plane of the head. Hence 
it is possible that a difference of phase in the ears may suggest 
to the mind that the sound is coming obliquely. 
12. — Since these results were worked out. Prof. Steinhauser 
has published a theory of Binaural hearing, in which he has 
worked out on geometrical principles the laws which determine the 
relative intensity with which a sound will reach the two ears when 
starting from different points. The intensities are equal in the 
two ears when the source of sound is in the median plane, and is 
a maximum when in front, a minimum when behind the head ; 
since the ears are set angularly, so as to catch sounds from in 
front of the head, in planes, which determine, according to 
Steinhauser, the conditions of best hearing. The operation^ 
