Prof. S. P. Thompson on the Pseudophone. 389 



Suppose one flap to be adjusted at an angle of about 40 

 degrees with the line of sight, in which position it is about 

 most favourably situated to receive sounds from a point right 

 in front of the observer; then if the other flap be adjusted to 

 any angle greater or less than 40°, fewer rays of sound are 

 reflected into the ear on that side than on the other ; and the 

 hearer imagines the source of sound to be situated on that side 

 on which the sensation is more intense. Accordingly, to verify 

 the perception, the hearer turns his head until both ears hear 

 the sound equally loudly, and imagines then that he is looking 

 in the direction of the sound, whereas he is looking at a point 

 situated nearer to that side on which the larger effective sur- 

 face exists. This observation agrees with Steinhauser's theory. 

 The illusion is very easily obtained by means of a loud-ticking 

 clock, but with some persons does not succeed unless their 

 eyes are blindfolded ; for when there is a conflict between the 

 evidence of the eyes and the evidence of the ears, the tendency 

 appears to be to believe the former rather than the latter. 



A more striking illusion occurs when the flaps of the pseu- 

 dophone are reversed and adjusted so as to reflect into the ears 

 sounds which come from immediately behind the observer. 

 In this case also, if a source of sound, situated anywhere be- 

 hind the head, be observed, if the observer does not know how 

 the flaps are adjusted, he will estimate it to be somewhere in 

 front; and, on turning his head about until the sounds are 

 equally intense, he judges himself to be looking straight at the 

 source of sound, whereas it is in reality exactly in an opposite 

 direction. This illusion succeeds well with a loud-ticking 

 clock, well also with the human voice, but not well with a 

 tuning-fork of medium pitch. In a room the experiment may 

 succeed with a tuning-fork ; but there is never the same clear 

 and decisive impression as to the position of the sounding 

 body. In the open air the writer has never succeeded in pro- 

 ducing the illusion with a tuning-fork ; for the sensation is one 

 of a character from which it appears to be impossible to draw 

 any precise judgment. The sound does not appear to have 

 any precise locality. This result, which agrees with some 

 experiments made by Lord Rayleigh with tuning-forks, stands 

 in strong opposition to Steinhauser's theory, which ought, if 

 true at all, to be a fortiori true for simple sounds. The author's 

 experiment differs from that of Lord Rayleigh in this respect — 

 that in the case of Lord Rayleigh' s experiments with the un- 

 aided ears the head of the observer was to be held immovable; 

 whereas in the experiment with the pseudophone the head is 

 turned about, seeking in vain a direction which can be pro- 

 nounced to be that of the sound-rays. 



Phil Mag, S, 5. Yol. 8. No. 50. Nov, 1879. 2D 



