386 Prof. S. P. Thompson on the Pseudophone. 



influences the perception of the direction of a sound in a sin- 

 gular manner. Moreover, since the sounds issuing from a 

 point to the right or left of the hearer travel paths of unequal 

 length to the two ears, the difference of phase thereby caused 

 in the two perceived sounds will depend also upon the wave- 

 length of the sound or on its pitch, if simple ; and the result 

 will be still more complex if the sound be not a simple tone. 



Again, Lord Rayleigb has shown reason for thinking* that 

 the diffraction suffered by the waves of sound as they travel 

 round the head of the observer to the two ears will affect 

 sounds of high and low pitch very unequally, and will there- 

 fore still further complicate the perception of the direction of 

 a sound by causing the quality of the sound, if compound, to 

 vary with the position of the head with respect to the direction 

 of the sound-waves, since in different positions the intensities 

 of the high and low components would be differently affected 

 as to their intensity. 



These considerations tend to throw some doubt upon the 

 reasonableness of the assumption made by Prof. Steinhauser, 

 in referring the perception of the direction of sounds to the 

 perception of their relative intensities. His conclusions are 

 in fact too general, and can only be considered applicable to 

 certain cases not complicated by questions of pitch or quality, 

 or by the influence of diffraction. 



In order to obtain a definite idea of the degree of trustwor- 

 thiness of the results so carefully elaborated by Prof. Stein- 

 hauser, the author undertook a series of experiments on the 

 perception of the direction of sounds of different kinds and 

 pitches, which are not yet concluded — but which at present 

 tend to show the unexpected result that Steinhauser's theory 

 is approximately true for sounds of high pitch only, and not 

 for sounds of medium or low pitch, and that it is more nearly 

 true for sounds in front of or behind the observer than for sounds 

 which reach him obliquely from right or left. 



In the course of these researches, it occurred to the author 

 that a simple means of testing some of the main features of 

 the theory was afforded by the possible production of acoustic 

 illusions, making sounds appear to come from other directions 

 than the real source. For if the perception of the direction of 

 a sound depends upon the relative intensities with which it 

 reaches the two ears, and if the intensities wdth which the sound 

 is perceived in the two ears depend, as Steinhauser assumes, 

 upon the effective magnitude of the pinnae or external flaps of 

 the ears and upon the angles which they make with the line of 

 sight, then any device which should virtually alter either the 

 * ' Transactions of Musical Association,' 1876. • 



