ON INSTRUMENTAL AIDS FOR DEAFNESS 263 



of times, and the sound correspondingly intensified. The 

 instrument is, in fact, a portable telephone-receiver and trans- 

 mitter. The possibility of doing this completely satisfactorily 

 depends on the facts (1) that the diaphragm of the telephone 

 receiver being of fairly thin sheet-iron, and of considerable 

 diameter, has an intrinsically low-pitched fundamental, (2) that 

 the partial tones it possesses are almost unlimited in number, 

 and do not form a " series " 1,2, 3, 4, etc., or 1, 3, 5, 7, etc., 

 with wide intervals of pitch as in ordinarily vibrating bodies ; 

 but that the higher members of the group have tones lying so 

 close together that the limited number of partial tones in the 

 voice sounds are pretty certain to have their counterparts in 

 those of the telephone diaphragm. There is a certain alteration 

 of the timbre, in part due to the representation of these partial 

 tones not being complete, but probably principally because the 

 changes of resistance of the microphone receiver are not pro- 

 portionate to the movement of its diaphragm, and as in the 

 telephone receiver the consonants are imperfectly produced. 

 Much improvement in this latter respect is doubtless possible 

 by the employment of some of those principles used in con- 

 nection with wireless telephony, and in fact much more rapid 

 improvement in various forms of apparatus would be feasible 

 if the persons who devise them had a modicum of knowledge 

 of physics, instead of being guided by pure empiricism. 



In brief, of the mechanical aids so far devised, classes B, 

 C, D, and E are of little efficiency, A is serviceable with the 

 limitation that it can only be used for conversation between 

 two persons, while class F is the most efficient. 



