454 Prof. More and Dr. Fry on the Appreciation 



waves in the same phase, the impression on the mind was 

 quite different from that when the disks vibrated oppositely. 

 But no experiments, had then been published which showed 

 that our power of locating sounds is largely due to this phase 

 interpretation*. 



There are three reasons which support this theory : first,, 

 experiments made on animals ; second, the fact that we can 

 locate complex tones made in the medial plane between the 

 ears; third, the systematic experiments given below. 



It is a significant fact that the ears (certainly at least the 

 outer parts) of many animals which have need to locate 

 sounds accurately, are placed on top of the head and close 

 together. This position is such that the head can make 

 absolutely no sound shadow unless the source of the sound is 

 directly below them. A sound coming from a considerable 

 distance to a horse must affect each of his ears with the same 

 intensity. Yet a horse, or any other of the animals tried, 

 had no difficulty in locatmo- the direction of a noise. This 

 can be readily proved. If a person stations himself behind 

 and somew r hat to the rigid of an animal and then whistles, 

 the animal invariably moves his head or ears to the right and 

 continues the motion until pointing directly to the person, 

 although he may be hidden. Of course the same holds true 

 for experiments made on the left side. The results are quite 

 certain, that such animals locate sounds accurately and yet 

 have no means of doing this by comparing the intensity of 

 the sounds in the two ears. 



It is well known that a pure tone made directly in front 

 of a blind-folded person cannot be distinguished from a similar 

 tone sounded directly behind. But one has not much difficulty 

 in distinguishing complex tones under like circumstances, 

 and especially is this true when the tones are made by the 

 human voice. As neither a difference in intensity nor in 

 phase exists, w T e were led to the belief that a complex tone 

 produced a different sensation according to the direction the 

 weaves entered the outside ear. Certainly the shape of the 

 outer ear is w^ell adapted to modify the short waves of the 

 higher harmonics, and thus to change the character of complex 

 tones. 



Lastly, we devised a set of experiments to see if the ap- 

 parent direction of a simple tone could not be changed by 



* The preceding' remarks are, of course, no longer correct, as Lord 

 Rayleigh, in bis Sidgwick lecture of last fall and in his paper in the 

 February issue of this journal, states his unqualified conviction that 

 phase-differences do affect the hearing. But, as they give the views held 

 at the time our experiments were made find in fact were the cause of 

 their being mode, we decided not to alter them. 



