CONTRIBUTIONS OF PHYSICAL SCIENCE 43 



most useful and effective of the new applications of physics 

 to the purposes of war. The second was responsible for the 

 location and destruction of thousands of enemy guns, while 

 the first was responsible for the location and destruction of 

 submarines, airplanes and mines. 



The binaural principle itself was unknown even to most 

 physicists before the war, though it is used by all of us when 

 we turn our heads until we think we are looking in the direction 

 from which a sound comes. The accuracy with which this can 

 be done in the absence of disturbing reflections is surprising. 

 When the observer has set his head so that the sound pulses 

 from the source strike the two ears at exactly the same time 

 he has the sense that the source lies in the median plane be- 

 tween the two ears. If the sound pulses strike one ear first, 

 the observer has the sense that the source is on the side of the 

 ear which is struck first. This sense is not due in any ap- 

 preciable degree to intensity differences produced by shadow 

 effects of the head. It has to do practically entirely with phase 

 differences. The principle is beautifully illustrated by insert- 

 ing into each ear one end of a piece of rubber tubing four or 

 five feet long and scratching or tapping on the wall of the 

 tubing, first at a point slightly closer to one ear than the other 

 and then moving the tapping object slowly through the mid- 

 point to a position nearer the second ear. The sound of the 

 scratching or tapping will then appear to the observer to be in 

 the ear which is nearest to it and then to move around the 

 head to the other ear as the mid-point is crossed, With the 

 unaided ear one can locate direction in this way to within five 

 or ten degrees. The simplest way to increase the sensibility of 

 the method to faint sounds is to increase the size of the ears 

 by providing them with trumpet-like extensions. To increase 

 the accuracy of location one stretches out the receiving ends 

 until the distance between them is say, five or six feet instead 

 of five or six inches, as it is in the case of the unaided ear. 

 It is then only necessary to turn the whole receiving system 

 through about one-twelfth the former angle to obtain the same 



