Dr. Alison on the Differential Stethophone. 387 



both ears, as in ordinary hearing. But if one cup be removed a Uttle, 

 say a half or a quarter of an inch from the watch (for we shall now 

 adopt it), and the other cap be left upon the watch, the sound is heard 

 with that ear only which is connected with the cup placed upon the 

 watch, and the sensation in the hearing ear is so marked, as to leave 

 the mind in no doubt whatever that it is through that ear we become 

 conscious of the sound. If the cup placed upon or nearer the watch, 

 be removed a little further than the other cup, so as to be less favour- 

 ably situated for collecting sound, say one inch from the watch, the 

 ear connected with it becomes totally unconscious of sound, and the 

 sensation of hearing is most unequivocally felt in the ear (and in that 

 ear only) which but a moment before was utterly deaf to it. If one 

 cup be placed upon the middle of the watch, and the other on the 

 edge, the watch sound is heard in that ear only which is connected 

 with the cup placed upon the middle. 



These experiments may be thus varied ; and the result will in 

 reality be the same, though apparently more remarkable. The watch, 

 being held in the air, at the distance of about an inch from one ear, 

 is heard distinctly beating into that ear only ; but if the watch be 

 now connected with the collecting-cup of the tube of the stethophone, 

 inserted into the other ear, the sound, being greatly magnified, is 

 heard in this ear, and in it only, the ear in whicji the sound had 

 been primarily heard being now altogether insensible to it, or un- 

 affected by it as far as our consciousness is concerned. The sensa- 

 tion of sound is transferred from one ear to the other, although the 

 watch is allowed to remain in close proximity to the ear that is now 

 deaf to its sound. 



A watch placed upon or inside the cheek, is heard to beat in that 

 ear which is nearer ; but if the opposite ear be connected with it by 

 means of one of the arms of the stethophone, or by a common flexible 

 stethoscope, the watch sound is no longer heard in the ear nearer the 

 watch, but in the ear further from it, which is now in reality brought 

 into nearer connexion with it, by means of the hollow tube. 



Sounds, produced in whatever material, are alike subject to this 

 law, so far as my experiments have j'et been made. 



The medium m which sounds are produced does not alter this law. 

 A watch ticking, or a bell ringing, either in the air or under water, 

 affords the same results. 



Sounding bodies give the same results when covered with soft or 

 hard materials. A watch placed in one corner of a box, a few inches 

 square, and an inch deep, is heard to beat in that arm of the ste- 

 thophone only which is near it. By this means, and by successive 

 movements of the instrument, and by attending to degrees of inten- 

 sity, the exact position of the watch may be with certainty indicated. 

 Or this may be effected by successively excluding those parts which 

 fail to cause hearing in one of the ears. 



The interposition of a body calculated to obstruct the sound at its 



entrance into one of the cups of tlic stethoplione, causes the sound to 



be heard in that car only wliich is connected with the cuj) which 



remains free from obstruction. This admits of ready jiroof, by 



2C2 



