10 PROCEEDINGS OP THE AMERICAN ACADEMY 



and it was found that the liability of such an occurrence could be 

 greatly diminished by covering the end of the stem with a layer 

 of wax. 



To make sure that these beats were not the result of imagina- 

 tion, we took forks giving four, eight, twelve, and sixteen beats 

 per second at random, struck them lightly as before, but in this case 

 held only one of them against the teeth, so as not to know the 

 number of beats which ought to be produced. When this fork had 

 ceased to be heard, we touched both forks to the wax in the ears, as 

 before, and in every case the correct number of beats was at once 

 heard. The experiment was varied slightly as follows. One ear 

 only was closed with wax; the other was immersed in a large basin 

 of water. The experiment was then repeated as above, with the 

 difference that one fork, instead of being touched to the ear, was 

 touched to the marble basin, its vibrations being transmitted to the 

 enclosed ear through the water. The same results were obtained 

 as before. These experiments were all carried on at night, when 

 there was not the slightest disturbing vibration in the air. To- 

 gether with another experiment described below, they lead us to 

 conclude that aerial vibrations acting upon the ear are not trans- 

 mitted through the skull or bony parts of the head from one ear to 

 the other. The experiments cited certainly seem to bear out this 

 view, at least for tones of feeble intensity. Furthermore, the fact 

 that differential tones have not been produced binaurally leads us 

 to believe that it is true generally, even for very intense sounds, 

 that binaural beats do not result from sound conduction within the 

 head. 



In his experiments Thompson was never able to obtain differen- 

 tial tones unless the sounds were allowed to mingle before falling 

 upon the ears. Our experiments fully bear out this result, which 

 we have verified in the following ways: (1) By listening to TJt± 

 and Mii forks through the telephones. These notes were trans- 

 mitted with more than sufficient intensity to give rise to a differ- 

 ential when heard in the ordinary manner; but with a telephone 

 placed at each ear, not the faintest trace of such a tone could be 

 heard. We also reached equally negative results on listening to 

 M^ with a telephone, and holding an Ut 4 , Sol if or Sol z fork close 

 to the other ear, or on leading their vibrations to the ear through a 

 long rubber tube. (2) By Thompson's method of carrying the 

 tones to the ears by means of rubber tubes (in our experiments 

 about thirty feet long), the forks being struck, one in another 



