1859.] on certain Auditory Phenomena, 69 



Wheatstone. A tuiihig-fork placed upon the middle of the forehead is 

 heard in that ear only that is closed. The closure prevents the escape 

 of sound, and promotes resonance to an extent sufficient to give the 

 restricting intensity. This restriction of hearing to the closed ear in 

 the case of sounds communicated through the bones of the head may 

 be imitated by an analogous closing of the apertures of the differential 

 stethophone. Sounds communicated in equal intensity to both limbs 

 of the stethophone at some point between the closed apertures and the 

 ear-knobs are heard louder than when the apertures are open ; and if 

 one aperture only be closed, the sounds are heard in that ear only that 

 is connected with the closed aperture. The wetted cotton of Mr. 

 Yearsley — really a cotton and water membrane — and the gutta-percha 

 membrana tympani of Mr. Toynbee, thus applied, greatly increase 

 sounds conveyed to the stethophone at points between the closed aper- 

 turesand the ear knobs. By closing one aperture with wetted cotton, 

 and the other with gutta-percha, the comparative value of these appliances 

 so far as intensification of sound communicated in the manner under 

 consideration, may be tested. Both of these appliances have been found 

 extremely beneficial in cases of deafness proceeding from perforate 

 membrana tympani. It would appear that it is by some closure of the 

 passages of the ear in persons partially deaf of one ear, that these per- 

 sons hear sounds communicated through the bones of the head, in that 

 ear only that is deaf; a fact ascertained in an extensive enquiry instituted 

 by the author, and that had been previously observed in a few cases. 



When one cup of the differential stethophone is held decidedly nearer 

 a sounding body than the other cup, the sound is perceived, as it were, 

 in the ear connected with the nearer cup ; but if the further cup be 

 brought somewhat towards the sounding body, so as to obtain more 

 sound, and to be more upon a par with the other cup, the sensation 

 changes its seat and is felt less in the ear and more towards the centre 

 of the head, or the spot mid-way between the two ears. This centri- 

 petal character of sensation is more marked as the two cups attain to a 

 parity ; and when this is fully accomplished sensation is located at a 

 central spot. If the cup, which was at first further from the sounding 

 body, be now gradually brought nearer than the other cup, a further 

 transition of sensation is produced ; it leaves the central spot and moves 

 towards the other ear, and becomes exclusively located theie, as 

 it was exclusively located at the first part of the experiment in the other 

 ear. By alternately and rapidly bringing the cups nearer the sounding 

 body, this movement of sensation may be rendered very striking. 



Lastly, the differential stethophone affords an unfailing test of the 

 existence of differences of intensity of sounds communicated by different 

 bodies, solid, liquid, or gaseous. If consecutive trials be required on 

 two bodies, this is done by using the two cups in succession, the 

 necessary movements being effected without changing the position of 

 the head, and with only a very little motion of the fingers. The con- 

 trast is readily made, and the difference, if any, with facility discovered. 

 If both limbs of the instrument be simultaneously employed, and if the 

 difference in the amount of the same sound conveyed to the instrument 



