the two Ears in the Perception of Space. 411 



Musical Association. Lord Bayleigh pointed out that, as the 

 size of the head bears a finite proportion to the size of sound- 

 waves, there will be no sharp shadows of sounds occurring at 

 the opposite side of the head to that at which a sound arrives, 

 but that diffraction of the sound-waves will take place, which, 

 in the case of complex sounds, will produce the result that the 

 partial tones of different pitch will arrive at the side of the 

 head opposite to that nearest the source of sound with very 

 different intensities. Thus, without making any assumption 

 as to the functions of the pinna? as resonators, Lord Rayleigh's 

 theory agrees with that of Mach in attributing the acoustic 

 perception of direction to differences of quality or timbre 

 between the two ears, the brain drawing, from the slight dif- 

 ferences of the tones received in the two ears, an unconscious 

 judgment based on empirical observation. 



7. One other theory only will be mentioned, and this 

 only to be at once dismissed. It is the theory advanced by 

 Kiipper *, that sound-waves proceeding in different directions 

 affect different parts of the tympanum, and so give rise to dif- 

 ferent sensations. This is physically untenable, because the 

 tympanum is not the true receptive organ with differentiated 

 nerve-structures, but only a part of a mechanical device by 

 which the alternating compressions and rarefactions taking- 

 place at the bottom of the tube of the ear are conveyed to the 

 true receptive organ in the labyrinth of the internal ear. It 

 is also untenable because, as Mach and Fischer have shown, 

 the outer ear cannot act as a true reflector except for waves 

 whose length is a small fraction of its own dimensions, which 

 is only true for sounds of such excessive shrillness as to be 

 out of the range of ordinary sounds. [The shrillest sound 

 audible in human ears is of wave-length 04 of an inch.] It 

 is, finally, untenable because, in fact, nothing is more difficult 

 than to tell the direction of sounds whose source is in the 

 median plane of the head — front, back, zenith, and nadir being 

 usually undistinguishable except for very w r ell-known sounds. 



8. In order to test the rival theories of binaural hearing, 

 the author of this paper devised a little instrument, described 

 before the British Association (Section A) in 1879 under the 

 name of the pseudopjlxone f, which is for the ears what Wheat- 

 stone's pseudoscope was for the eyes, an instrument for veri- 

 fying the laws of perception by means of the illusions which 

 it produces. The pseudophone consists merely of a pair of 

 adjustable flaps or reflectors which can be fitted to the ears, 

 and which can be set at any desired angle to catch sounds 



* Arc/a'v fur Ohrenheilkundc, u. F. ; Bel. ii. part 3, p. 158. 

 t Phil. Mag. [5.] vol. viii. 1879, p. 385. 



