Dr. Alison on the Differential Stethophone. 389 



direction of the same sound, with more accuracy than could be done 

 had a judgment to be formed between the intensity of two similar 

 sensations in the two ears respectively. All source of error is 

 removed by there being only one sensation, although there may be 

 two impressions. This law of a stronger impression in one ear, 

 rendering us unconscious of a weaker but similar impression in the 

 other, has an analogue, though perhaps an imperfect one, in the 

 sense of touch. Very strong impressions upon one part of the 

 body cause such acute sensations, that minor impressions of the 

 same kind upon another part are frequently not felt, in fact, produce 

 no sensation. 



The only observations bearing upon this law which I have been 

 able to discover, are some by ]\Ir. Wheatstone, in a paper entitled 

 " Experiments on Audition," published in the ' Quarterly Journal of 

 Science, Art, and Literature,' vol. ii. New Series, 182/. These expe- 

 riments are intended to show the augmentation which the sensation 

 of autophonic sound, and the sounds of a tuning-fork applied to the 

 head, acquires when the ear is closed, although the perception of 

 external sounds is diminished. ^Ir. Wheatstone shows that a vocal 

 sound is heard louder in that ear that is closed, say with the finger, 

 than in the other. He also shows, that the sound of a tuning-fork 

 placed upon the head is heard louder in that ear wliich is closed than 

 in the other which remains open, even though the tuning-fork may 

 be brought nearer the open ear than the closed one. These experi- 

 ments, Mr. Wheatstone says, prove that " sounds, hiunediatelj/ com- 

 municated to the closed meatus externus are very greatly magni- 

 fied ;" and he adds, "it is an obvious inference, that if external 

 sounds can be communicated to act on the cavity in a similar man- 

 ner, they must receive a corresponding augmentation." 



This distinguished philosopher constrvicted tlie instrument named 

 a Microphone, for the purpose of augmenting weak sounds, upon this 

 principle, i. e. the augmentation of sound by closure of the ears ; 

 and he informs us that it " is calculated for hearing sounds when it 

 is in immediate contact with sonorous bodies," and that "when they 

 are diffused by their transmission through the air, this instrument 

 will not afford the slightest assistance." This instrument is spoken 

 of in connexion with the augmentation of sound, and not in reference 

 to the limitation of sound to one ear, or to the comparison of sensa- 

 tions in the two ears. The remarkable, and, to the uninitiated mind, 

 the wonderful fact, made known more than thirty years ago by 

 Mr. Wheatstone, that a tuning-fork held upon the head close to an 

 open ear is not heard in this ear, but in the opposite ear, provided it 

 be closed with the finger, or by some other means, proved that 

 sounds connnnnicatcd to the skull were exclusively heard in the 

 closed ear. In the case of the tuning-fork, the fact made known by 

 Mr. Wheatstone is undoubted. The rationale of the phenomenon 

 appears to be- this : — The vii)rations of the tuning-fork .are commu- 

 nicated to the bones of the head, and through them to the cars 

 including their bones, cartilages, and contained air ; but in the case 

 of the closed ear, the vibrations arc permitted no egress or escape 



