1896 - 97 .] Dr Albert A. Gray on Direction of Sound. 445 
the whole matter. Why are we able to localise a high note or a 
compound note having partial tones better than a low pure tone, 
a fact which experimenters are agreed upon (Mach, Lord Ray- 
leigh, Dr P. Thompson). 
At a meeting of the British Association in August 1877, Prof. 
S. P. Thompson read a paper in which he showed that the ear, 
or, rather, the two ears acting together, are enabled to distinguish 
differences of the phase of sound. The same fact was discovered 
independently by Lord Kelvin a few months later. In the 
Philosophical Magazine for June 1882, Prof. Sylvanus Thompson 
had a paper in which he suggested the possibility that differences 
of the phase with which a sound affects the two ears may aid in 
determining its direction. 
Thanks to the kindness of Prof. MacKendrick, I was enabled 
to try these experiments myself. One of them was particularly 
interesting. I selected two ut 4 forks, and loaded one so as to 
give slow beats when both were sounded. They were placed 
several yards from one another, and a tube was led from the close 
proximity of one into the right ear, and another tube was led from 
the other fork into the left ear. As the original experimenter 
noticed, beats were heard very distinctly ; similarly, Prof. Thomp- 
son noticed that although beats were heard yet the note seemed 
continuous. For my own part, I was able to observe the same 
strange phenomenon, but in my case it required an effort of the 
will ; by listening very intently for the note I was enabled to 
hear it absolutely continuously, but on relaxing the attention it 
seemed interrupted by the beats. On the other hand, the beats 
obtruded themselves upon the mind, no matter how intently I 
listened for the tone and neglected them ; there was no getting 
away from them. 
Some months ago, while attempting to find out by what means 
the two ears acting together are able to appreciate difference of 
phase, I discovered a peculiar fact. Wishing to produce what 
may be termed a continuous positive phase in one ear while a 
sound was conducted to the other, I selected an intelligent 
individual possessed of normal hearing power. The tympanic 
membrane of one ear was illuminated in the usual way by 
reflected light and a speculum, while a tuning-fork was held a 
