Perception of Sound Direction. 231 



presumed that this failure is connected with obtuseness to 

 sounds of high pitch, such as occur especially in the sibilants. 

 For some years I have been aware that I could no longer hear 

 as before many of the high notes from bird-calls, such as I 

 employ with sensitive flames for imitating optical phenomena. 

 If. as seems the only possible explanation, the discrimination 

 of front and back depends upon an alteration of quality due 

 to the external ears, it was to be expected that it would be 

 concerned with the higher elements of the sound. In this 

 matter it would not be surprising if individual differences 

 manifested themselves, apart from deafness. A " paddle- 

 box " formation of the external ear, if not ornamental, may 

 have practical advantages. 



My assistant, Mr. Enock, is able to make discriminations 

 between front and back, though I think not so well as I 

 used to be able to do. Experiments of this kind are easily 

 tried on a lawn in the open, the observer closing his eyes 

 and ears, with if necessary a movement of the thumbs over 

 the latter to drown residual external sounds. At the moment 

 of observation the ears are of course opened. In observing 

 sounds from sources not conveniently moveable, such as the 

 ticking of a clock, the rotating stool is useful. 



As had been expected, Mr. Enock's judgement was liable 

 to be upset by the operation of little reflecting flaps situated 

 just outside the ears. The arrangement was that of Prof. 

 Thompson's " pseudophone" *, whereby the reflectors, whose 

 planes were at an angle of 45° with the line of the ears, 

 could l)e rotated in a manner unknown to the observer about 

 that line as axis. In my use of it the two reflectors were 

 always adjusted symmetrically. Thus, if the reflectors 

 were so turned as to send into the ears sounds from the front, 

 no mistakes were made, as if the action were co-operative 

 with the natural action of the external ears. On the other 

 hand, if the collars carrying the reflecting flaps were turned 

 through 180° so as to reflect into the ears sounds from behind, 

 frequent mistakes ensued. I hope before long to be able to 

 confirm and extend these observations. 



In conclusion, I will remark that the facts now established 

 have a possible practical application. In observing fog- 

 signals at sea it is of course of great importance to be able 

 to estimate the bearing. If a sound is of sufficiently long 

 duration (5 or 6 seconds), it is best by turning the body or 

 head to bring it apparently to the right and to the left, and 

 to settle down into the position facing it, where no lateral 

 effect remains. If, as for most fog-signals, the duration be 

 * Phil. Majy. November 1879. 



