ON INSTRUMENTAL AIDS FOR DEAFNESS 



By PROF. F. WOMACK, B.Sc, M.B. 



The question has often been propounded — why is it apparently 

 not possible to remedy defects of hearing, especially such as 

 are concurrent with advance of years, with the same satis- 

 factoriness that defects of vision can be remedied by the use 

 of spectacles ? In answering this question, we shall be better 

 able to understand the difficulty to be surmounted if we con- 

 sider the difference of the two scientific problems. 



Most of the difficulties or defects of vision are due, not to 

 lack of sensitiveness of the retina, but to limited range of 

 focussing power ; and all that is necessary is to so deflect the 

 path of the light by refraction as to cause the rays to diverge 

 from some point within the restricted range of focussing. Had 

 the defect been due to lack of sensitiveness of the retina, the 

 problem would be as difficult of solution as that relating to 

 hearing ; in fact, the former would be virtually unsolvable, 

 since the brightness of every image of finite magnitude is 

 diminished by every optical contrivance employing reflection 

 or refraction. The problem in the case of hearing is, how to 

 increase the energy density of the sound waves reaching the 

 observer — how to concentrate the vibrational energy of the 

 wave motion, that the number of ergs per second per square 

 centimetre may be sufficiently increased to reach the minimum 

 necessary for audition. The energy density necessary for 

 hearing by a normal person is surprisingly small. Dr. P. E. 

 Shaw measured the amplitude of movement of the diaphragm 

 of a telephone when this was only just sufficient to produce an 

 audible sound, and found that this minimum displacement was 

 of the order o"j x io~ 7 (seven hundred-millionths of a centi- 

 metre, or three hundred-millionths of an inch). The air 

 immediately adjacent to the telephone diaphragm would 

 however not move backwards and forwards even to this extent, 

 but as estimated by Lord Rayleigh would be approximately 



five times less, or 0*15 x io - ' cm. 



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