298 DYNAMICAL THEOKY OF SOUND 



of consonant triads, especially of simple tones, but enough 

 has been said to shew their importance from the musical point 

 of view. For further developments reference must be made to 

 the work of Helmholtz*. 



97. Perception of Direction of Sound. 



One important question of physiological acoustics in which 

 dynamical principles are involved remains to be mentioned. 

 An observer, even when blindfolded, and with no adventitious 

 circumstances to guide him, is in general able to indicate with 

 great accuracy the direction from which a sound proceeds. In 

 the case of pure tones the discrimination between back and 

 front is indeed lost, as was to be expected, considering the 

 symmetry with respect to the medial plane of the head, but 

 right and left are clearly distinguished. For tones of small 

 wave-length this may be accounted for by the difference of 

 intensity of the sensation in the two ears, since the head acts 

 to some extent as a screen, as regards the further ear. But 

 when the wave-length of the sound much exceeds the peri- 

 meter of the head the investigation given near the end of 81 

 shews that this difference must be very slight. According to 

 the most recent investigations of Lord Rayleigh^, the in- 

 terpretation depends on the relative phase of the sounds as they 

 reach the two ears, a difference of even a fraction of a period 

 being effective. He found that if the same tone be led by 

 different channels to the two ears, and all extraneous dis- 

 turbances be excluded, the sound can be made to appear to 

 come from the right or left at will, by adjusting the relative 

 phase. The origin of the sound was always attributed to that 

 side on which the phase is in advance (by less than half a 

 period). The result, which has been arrived at independently 

 by other observers, is at present unexplained. It has been 

 suggested that the phenomena may really be due to a differ- 

 ence of intensity. A fraction of the sound may be transmitted 

 from each side to the opposite internal ear, through the bones of 



* See also Sedley Taylor, Sound and Music, London, 1873. 

 t Phil. Mag. (6), vol. xm. (1907). 



