BIN AURAL AUDITION. 1163 



tuning-fork of fairly high pitch be held a few inches from one ear, while 

 the chain of ossicles of the other ear is pushed gently inwards by a 

 probe or by pressing the finger into the meatus, thus condensing the 

 air within, the tuning-fork or watch will be better heard. This is prob- 

 ably due to increased labyrinthine pressure in one ear, causing a reflex 

 contraction of the intra-tympanic muscles of the opposite ear. 



Urbantschitsch l has stated that the stimulation of one ear by sound 

 causes increased sound perception in the other ear. This may be true 

 under some circumstances, but certainly not under all, for, as Politzer 2 

 says, a loud sounding tuning-fork held opposite one ear destroys the 

 perception for a weak sounding fork held opposite the other ear. 



Various writers have recorded cases of subjective colour sensations 

 accompanying audition, and these cannot be considered pathological, for 

 they have occurred in perfectly healthy individuals. 3 



BINAURAL AUDITION. 



Binaural audition, in the ordinary sense of the term, does not affect 

 the quality of sounds, but there is little doubt that it does affect 

 the intensity with which sounds are perceived. With the exception 

 described above (p. 1163), we hear a sound louder with two ears than 

 with one. Urbantschitsch 4 attributes this to the passing of subjective 

 irritation from the auditory centre of one side to that of the other. 

 Politzer, 5 however, maintains that this explanation is superfluous, and 

 considers that the superiority of binaural over monaural audition is due 

 to the well-known physiological law of " summation of sense." 



But the greatest difference between binaural and monaural audition 

 is, that by the former we are enabled to judge of the direction of sound 

 with considerable accuracy, whereas in hearing with one ear we are 

 almost completely deprived of this faculty. Our judgments as to the 

 direction of sounds are formed, for the most part, from the different 

 degrees of intensity with which they are heard by the two ears. 



In 1874, Mach 6 put forward the theory that the pinnae of the ears 

 act as resonators for the higher notes, and that their resonating capacity 

 varies according to their position relative to the waves of sound. 



Lord Eayleigh 7 holds that diffraction of the sound-waves will 

 occur as they pass round the head to the ear remote from the source of 

 sound ; thus the partial tones will reach the two ears with different 

 degrees of intensity, and consequently there will be a difference in the 

 quality of the sound as heard by the two ears. From this difference in 

 quality we draw our inference as to the direction of the sound. 



S. P. Thompson 8 has advanced a theory somewhat similar to that 

 of Lord Eayleigh. His view of the matter is, that we estimate the 

 direction of a sound by the difference of intensity with which those 

 partials are heard which have the greatest effect in producing the 

 difference of quality with which the sound is heard by the two ears. 

 This hypothesis is in keeping with the fact that the direction of a 

 compound sound can be much more accurately estimated than the 

 direction of a simple note, particularly if it be of low pitch. 



1 "Lehrbuch d. Ohrenh.," 1890, S. 416. 2 " Diseases of the Ear," 1894, p. 608. 



3 Baratoux, "L'audition colorize," Paris, 1888 ; Binet, Rev. d. devx mondes, 1892. 



4 Loc. tit. 5 Op. tit., p. 608. 



6 Arch.f. Ohrenh., 1874, Bd. ix. S. 72. 7 Trans. Music. Soc., London, 1876. 



8 London, Edinburgh, and Dublin Phil. Mag., London, 1882, vol. xiii. p. 406. 



