Dr. Alison on the Differential Stethophone. 391 



inches. As the further cup is brought nearer and nearer the watch, 

 the sound, always confined to the more favoured ear, is gradually and 

 steadily intensified, until the two cups are, or are about to be, similarly 

 placed, at which moment the sensation ceases to be restricted to one 

 ear, and has acquired its greatest intensity. This fact proves, that 

 though the sensation of hearing be confined to the ear to which sound 

 is communicated with greater intensity, we profit by the sound which 

 is conveyed into the other ear, though failing to produce a sensation 

 or a consciousness of sound there, by its serving to augment very 

 materially the sensation of sound in the more favoured ear. The 

 less favoured ear thus augments the sensation which we experience, 

 at the same time that it fails to interfere with the aid which the 

 sensation confined to one ear affords us as to the direction of external 

 sounds. 



The sounds of which we have been treating as differently affecting 

 the two ears, according to the intensity with which they are respect- 

 ively communicated, are of the same character, though differing in 

 intensity. It is sounds of the same character only which exhibit the 

 phenomenon of restriction in virtue of moderately different intensity. 

 The sounds must emanate from the same sounding body, or from 

 bodies sounding similarly. A little difference in character will cause 

 the experiment of restriction to fail. 



Thus, if two bells, differing considerably in character, be rung re- 

 spectively in the two ears, one louder and graver than the other, the 

 louder and graver sound does not render the other ear insensible to 

 the weaker sound of the weaker bell. Both ears hear perfectly, but 

 the loud, grave sound is heard iu one ear, and the weak sound is 

 heard in the other. 



If, instead of one watch, we place two together, having sounds of 

 different character, as for instance one low and grave, and another 

 loud and sharp, and the two arms of the stethophone be placed over 

 them respectively, the sounds of both watches are heard, but the 

 sound of one is heard in one eai', and the sound of the other is heard 

 in the other ear. The loudness of the sound in one ear does not 

 increase the weakness of the sound in the other ; or, in other words, 

 the intensity of the sensation produced by the weak watch in the one 

 ear is not reduced by the sensation produced by the loud watch in 

 the other ear. 



The sound of a watch ticking continues to be heard in one ear, 

 although a large-sized bell is made to ring at the other ; and I have 

 not perceived that the sensation produced by the watch is at all 

 imj)aired by the bell. A whistling lung-sound heard in one ear, is 

 not rendered less obvious by a loud blowing lung-sound in the other. 

 A hissing murmur at the apex of the heart conveyed into one ear, 

 and a rasping sound at the base conveyed into the other, arc both 

 heard without alteration in the cars to Avhich they are respectively 

 conveyed . 



IJy virtue of these two laws — 1st, that sounds of the same cha- 

 racter are restricted to that ear into which they arc conveyed iu 

 greater intensity, and 2nd, that sounds diilcring in character may 



