OF ARTS AND SCIENCES. 9 



stated, on account of the remarkable intensity with which their 

 tones were transmitted, this being approximately the natural tone 

 of the diaphragm. The singular phenomena of the localization in 

 the back of the head of the sensation caused by two notes in oppo- 

 site phases when heard by separate ears, and the apparent wander- 

 ing of the sensation from one ear to the other in the case of binaural 

 beats referred to by Thompson, were always clearly recognized. 



The method of experimentation precludes any fusion of the two 

 sounds, except within the head; but to decide whether the beats 

 arise from an interference of the sensations or from a mechanical 

 interference in Corti's organ, we must investigate further. We 

 tried to throw some light on this point by the following experiments. 

 It is well known that the vibrations of a fork, even when of too 

 small amplitude to be heard when held opposite the ear itself, can 

 be made audible by pressing the stem of the fork against the bone of 

 the skull, or, still better, against the teeth, the latter transmitting 

 vibrations to the ears better than any other part of the head. A 

 still more effectual means of making audible very small vibrations is 

 to close the ear with a bit of beeswax and press the stem of the fork 

 lightly against the wax. In this case the vibrations are transmit- 

 ted to the membrana tympani by the small amount of air enclosed 

 within the meatus, as is clear from the fact that the sound of the fork 

 is heard on touching the wax long after it ceases to be audible on 

 touching its stem to the pinna of the ear. Hence in this case there 

 is no conduction to the middle or inner ear through the bones of the 

 head. Whatever sound reaches the ear follows the ordinary path 

 through the meatus to the membrana tynipani. Now we found 

 that the vibrations of a fork could be heard longer when touched to 

 the wax in the ear than when held against the teeth. We therefore 

 took two small tonometer forks making four beats per second, struck 

 them very gently, and held their stems against the teeth; loud beats 

 were heard in the ears just as they are ordinarily heard when power- 

 ful forks are sounded in the air. The forks were then held in this 

 position until the beats had entirely ceased to be audible, when 

 they were removed, and the stem of each was touched to the wax 

 closing the two ears. Instantly the two notes were heard, faintly 

 but distinctly, in the ears to which they were held, and accompany- 

 ing them were faint beats seeming to wander in the head from ear 

 to ear, as is always the case with binaural beats. In trying this 

 experiment care was taken that no vibration should be imparted to 

 the fork from a gentle blow given on touching its stem to the wax, 



