4.10 Prof. S. P. Thompson on the Function of 



ence of phase, being thus recognizable, might assist the observer 

 in finding the true direction of a sound ; for when his gaze 

 was directed in the precise direction, not only would the rela- 

 tive intensities be equal, but the phase would be identical, and 

 the sensation would be free from complication with the 

 illusion. 



Prof. Alfred M. Mayer of the Stevens Institute, in the 

 course of his acoustic researches, suggested an instrument 

 which he called the topophone, the object of which is to ascer- 

 tain the direction of a source of sound, and which depends upon 

 differences of phase and of intensity. An experiment of Zoch 

 (see Radau, Die Lehre vom Schall, p. 246) for comparing the 

 relative phases of two simple unison-tones by means of Konig's 

 manometric flames, suggested to Mayer an instrument com- 

 posed of two resonators fixed at the ends of a T-shaped frame, 

 and connected with manometric flames, whose objective vibra- 

 tions, as seen in a rotating mirror, would be synchronous when 

 the phases and intensities of the sound-waves received in the 

 two resonators were alike, or when the cross-arm on which 

 they were fixed stood at right angles to the direction of the 

 sound. In another form of the instrument, the frame carrying 

 the resonators was fixed by a kind of yoke to the shoulders, 

 the resonators being respectively connected by tubes to the 

 two ears. Comparatively small angular movements of the 

 head would then produce considerable differences of phase, 

 rendering the perception of direction more narrowly accurate. 



6. Theories of Mach and of Lord Rayleigh. — Prof. Mach, 

 of Prague, advanced the theory (Archiv fur OhrenheiUaindr. 

 1874) that the perception of direction of sound arose from the 

 operation of the pinnse of the ears as resonators for the higher 

 tones to be found in the compound sounds to which the ear is 

 usually accustomed ; their action as resonators should be more 

 or less effective according to the position of the pinna? with 

 respect to the direction of the sound-waves; and by thus rein- 

 forcing in different positions with unequal intensity, some one 

 or more of the higher tones of a compound sound should affect 

 the quality or timbre of the perceived sound, producing a dif- 

 ference between the sounds heard in the two ears in all posi- 

 tions, save when the source of sound lay in the median plane 

 of the head. There should also, according to this theory, 

 be a difference in the quality of the perceived sound (owing 

 to the dissymmetry of the upper and lower parts of the pinna) 

 for sounds in the median plane according as they were above, 

 below, or in the line of sight. It will be convenient to 

 consider along with the theory of Mach the souu^vhat similar 

 theory of Lord Rayleigh*, propounded in 1876 before the 

 " Trans. Mus. Assoc. 1870. 



