the two Ears in the Perception of Space. 413 



at an equal distance in front of the head. With most people 

 this angle is less ; but most people, at any rate, prefer to face 

 the source of sound when listening. 



(/) It may be deduced from Steinhauser's theory that the 

 perception of direction should be more accurately defined in 

 those persons for whom the angle between the effective plane 

 of the pinna and the line of sight is as small as possible. If 

 this be so, persons whose ear-flaps stand off from their heads 

 at a considerable angle should be bad judges of the direction 

 of sounds, which is not, in general, the case. The behaviour 

 of animals in bringing forward the openings of the pinnas 

 when listening intently to a sound is contrary to this point. 

 If the deduction were true, women should be better judges of 

 the direction of sounds than men, as their pinnse usually are 

 less prominent. In the experiments of Graham Bell with 

 telephone-receivers when they were set at an angle with one 

 another, he was able to perceive the direction of sounds more 

 accurately than when the receivers were set parallel, the dif- 

 ference between back and front being thus capable of discri- 

 mination; but he does not state whether this discrimination 

 arose merely from difference in relative intensity or from dif- 

 ference of quality. 



(g) If the theory of Steinhauser is true, the perception of 

 direction should be more accurate for sounds when their source 

 is far away than when it is near the head. If the theory of 

 Lord Rayleigh and of Mach is true, the perception of direc- 

 tion should be more accurate for sounds whose source is 

 nearer the head. The latter is shown by experiment to be 

 true for all kinds of sounds. 



(Ji) The theory of Lord Rayleigh is supported by the 

 experimental fact that the quality of sounds does, as a matter 

 of fact, differ when they are heard in different directions. 

 Compare, for example, the sound of a musically ticking clock 

 placed in front of the head with the sound of the same clock 

 when the head is turned in the opposite direction. To the 

 writer the tick of a watch always sounds deeper in pitch when 

 held in front of the head than when held opposite either ear, 

 in each case at equal distances of about one foot away. 



(i) The theory of Lord Rayleigh also derives support from 

 the following observation. To analyze a compound sound by 

 ear, so as to say what partial tones are present, requires consi- 

 derable practice, and is even then not easy. But the presence 

 or absence of partial tones is rendered much easier of detec- 

 tion if the head be turned about, so that their various degrees 

 of intensity as affected by diffraction or by phase-difference 

 may bring one or other into prominence. 



Phil. Mag. S. 5. Vol. 13. No. 83. June 1882. 21 



