200 



applying the two cups as much as possible equally on the middle of 

 a watch about an inch above it, and by placing two fingers held 

 together between one cup and the watch. When this is done, the 

 watch is heard to tick into the ear that remains free from obstruction. 



The removal of an obstructing body from one cup, while it is 

 allowed to remain in operation with the other, causes sound which 

 had been equally heard with both ears, to be heard in that one only 

 which is connected with the cup freed from the obstructing body. 

 Thus, if the fingers be interposed between the watch and the cups 

 held equally over it, and the fingers be separated under one of the 

 cups, so as to permit of atmospheric communication, the sound is 

 heard in that ear only which is connected with this cup, and not at 

 all in the other. 



The effect of intensification of a sound in one ear depriving the 

 other ear of all sensation of that sound, is interestingly shown by 

 placing the tubes of the instrument across a block of wood with the 

 cups hanging in the air. While both cups are left open, and a 

 tuning-fork in vibration is placed between the two tubes, the sound is 

 heard with both ears ; but if one cup be closed with the hand, or 

 with leather, and the other be left open, the sensation of sound is re- 

 stricted to that ear connected with the closed cup. The sound in the 

 tube connected with the closed cup is rendered more intense by the 

 closure, the escape of sound is obstructed, and reverberation takes 

 place. By virtue of the intensification, sensation is monopolized by 

 one ear, and is lost in the other. The result and the mechanical con- 

 ditions are much the same as in the experiments of Mr. Wheatstone 

 with a tuning-fork held upon the head, presently to be referred to. 



It is worthy of observation, that in order that a sound previously 

 heard with, or in both ears, as in the above experiments, may be 

 appreciated or felt in one ear only, it is not necessary that the 

 stethophone, or other conducting instrument, be placed in the cavity 

 of the meatus externus. It is sufficient for this result that the instru- 

 ment be placed near the meatus, so as to give it an advantage of 

 intensity over the opposite cavity. When the instrument is to 

 be held only near the meatus, care should be taken not to touch the 

 external ear, so that there may be no conduction by that part from 

 contact, which would vitiate the experiment. The result is perfectly 

 satisfactory and conclusive, although the remarkable sensation of 



