Prof. S. P. Thompson on the Pseudophone. 



387 



effective magnitude or the angle of the pinnse to an amount 

 unknown to the observer must produce a false perception of 

 the relative intensities of the sounds, and give rise to an illu- 

 sion as to the direction of the sound. 



The simple instrument for 

 which the author suggests the 

 name Pseudophone consists of 

 a pair of ear-pierces, A A, fur- 

 nished with adjustable metallic 

 flaps or reflectors of sound, C 0, 

 which can be fitted to the ears 

 by proper straps, D and E, and 

 can be set at any desired angle 

 with respect to the axis of the 

 ears, and can also be turned 

 upon a revolving collar about 

 that axis so as to reflect sounds into the ears from any desired 

 direction. 



The theory of the Pseudophone is very simple, and is as 

 follows : — 



The intensity of a perceived sound depends upon the amount 

 of space over which the waves are gathered by the external 

 collecting apparatus of the ear ; and by analogy with optical 

 phenomena we may say it depends upon the number of rays 

 of sound which reach the ear. 

 Let the effective surfaces of the 

 pinnae which gather the sound- 

 rays be /,_ and / 2 , and let them 

 make equal angles fa and fa with 

 the line of vision. Let the rays 

 of sound that reach the ears fall 

 in a direction indicated by the 

 lines S, S, S, making an angle 6 

 with the line of sight. The lines 

 m and n, which are drawn per- 

 pendicularly, measure the num- 

 ber of sound-rays which reach 

 the pinnae, and are therefore pro- 

 portional to the intensities of the 

 sounds which reach the ears. 



Now 



and 



m=/isin(0 + 0) ? 

 ™=/ 2 sin (</>-#); 

 m_ /i sin (# + (/>) 

 n / 2 sin(0— dy 



