Perception of Sound Direction. 225 



ends of the nerves along which the communication with the 

 brain is established. On the contrary, the processes in the 

 nerve must themselves be vibratory, not of course in the gross 

 mechanical sense, but with preservation of the period and 

 retaining the characteristic of phase — a view advocated by 

 Rutherford, in opposition to Helmholtz, as long ago as 1886. 

 And when we admit that phase-differences at the two ears of 

 tones in unison are easily recognized, we may be inclined to 

 go further and find less difficulty in supposing that phase- 

 relations between a tone and its harmonics, presented to the 

 same ear. are also recognizable. 



The discrimination of right and left in the case of sounds 

 of frequency 128 and lower, so difficult to understand on the 

 intensity theory, is now satisfactorily attributed to the phase- 

 differences at the two ears. The next observations relate to 

 pure tones of pitch 256. Two large forks of this pitch were 

 used, such as are commonly to be found in collections of 

 acoustical apparatus. Tuned with wax so as to give beats of 

 3 or -1 seconds period, they may be held (after excitation) by 

 their stalks one to each ear, preferably by an assistant. The 

 sensation of transference from right to left was fully marked, 

 and when the conditions were good, especially in respect of 

 equality of intensities at the two ears, the vdiole of the sound 

 seemed to come first from one side and then from the other. 

 The method of holding in the fingers is satisfactory as regards 

 the isolation of the sounds.. Practically nothing of either 

 fork can be heard by the further ear. But there is a little 

 difficulty in maintaining quite constant the relative positions 

 of forks and ears. 



In another arrangement, which has certain advantages, the 

 orks are mounted, stalks upwards, on a sort of crown, in such 

 a fashion that the free ends of the forks are about opposite 

 the ears. A sketch by Mr. Enock is reproduced in the figure 

 (p. 226). If the crown be sufficiently large, it can be adapted to 

 various sized heads with the aid of pads. At the back, attached 

 to the crown, is a forked tube of brass, symmetrically shaped, 

 whose open ends abut upon the faces of the forks. The short 

 limb, forming the stalk, is prolonged by india-rubber tubing 

 and so connected with the ear of the assistant observer. If 

 the forks are vibrating equally, a well-defined silence marks 

 the moment of phase-opposition. To excite and maintain the 

 vibrations, a violin-bow is employed in the usual manner. 



Very good observations may be made in this way if the 

 vibrations of the forks are equally and sufficiently maintained. 

 The assistant, listening throuoh the forked tube, is able to 

 give a sharp signal at the moment of phase-opposition. A 



