Prof. S. P. Thompson on Binaural Audition. 389 



the funnel, and then the fork be slowly rotated round its axis, 

 the vibrations transmitted down the other funnel will alter- 

 nately agree with or differ from those transmitted down the 

 first funnel, according to the aspect of the fork. Two forks 

 were chosen for the experiment, UT 3 and UT 4 (c' = 256 and 

 c" = 512). Both were set vibrating at the point whence their 

 vibrations could enter each funnel ; and they were placed at 

 first so that the vibrations of each fork reached the ears in 

 similar phases. The compound tone was heard "in the ears." 

 Then the UT 4 fork was rotated a quarter round on its axis, 

 so that it sent opposite vibrations to the two ears. The two 

 compound waves transmitted by the two tubes differed there- 

 fore like those of the diagram above. The sound of the UT 4 fork 

 moved its position to the back of the head ; the sound of the UT 3 

 fork remained localized in the ears. When the UT 3 fork alone 

 was turned round a quarter, its sound alone was localized at 

 the back of the head. When both were turned one quarter, 

 both notes appeared to sound at the back of the head. Here 

 is a case, then, in which by listening with the two ears we can 

 detect a difference of phase between the component partial 

 tones of a compound tone. And when we reflect that, except 

 for sounds whose origin is in the plane coinciding with the 

 median plane of the skull, the lengths of the paths by which a 

 sound reaches the two ears are unequal, so that the phases of 

 the vibrations received simultaneously by the two ears cannot 

 be alike for all wave-lengths, we must admit that this binaural 

 perception of a difference of phase cannot but be of importance 

 in the appreciation by the ear of the quality of compound 

 sounds such as those of musical instruments (in which every 

 fundamental tone is accompanied by a certain characteristic 

 series of upper partials), and of the sounds of the vowels. 



10. As to the cause of the singular localization discussed in 

 the preceding paragraphs, whether it be physical, physiological, 

 or psychological .(that is to say, purely associative), I hesitate to 

 give an opinion without further evidence. In searching for a true 

 explanation we must not lose sight of the other phenomenon 

 of the interference in the perception of sounds, though it is 

 by no means certain that both phenomena can be referred to 

 the same cause. There is no decussation of the auditory 

 nerves, like that of the optic nerves, to account for a blending 

 of the sensations. The portio mollis of the right does not in- 

 tersect or have any commissure with the portio mollis of the 

 left after leaving the fourth ventricle of the brain, from which 

 they originate. This point deserves the attention of anato- 

 mists and physiologists. 



It may be possible to explain the phenomenon of localiza- 



