ON INSTRUMENTAL AIDS FOR DEAFNESS 259 



service for the purpose of ordinary conversation between two 

 persons. The physical principle is merely that of continued 

 reflection of the sound waves from the inner wall of the tube, 

 with consequent prevention of spreading out of the wave front 

 and diminution of the energy density with increase of distance. 

 The law of reflection is immaterial, and the efficiency is but 

 little affected by the bore of the tube, which may be attributed 

 to conduction of sound also through the material of the tube 

 (as in a wooden stethoscope) playing some secondary part in 

 the process. 



Before considering the next classes, B, C, D, it may be as 

 well to refer to the underlying acoustic principles, viz. reflection 

 of sound waves and resonance. 



All reflection of wave motion by a surface to an observer 

 is fundamentally diffuse reflection ; that is, however large or 

 whatever the shape of the reflecting surface may be, every 

 part contributes something to the energy reaching the ob- 

 server. When we are dealing, however, with short waves, such 

 as those of light, the outlying portions of the surface reflect 

 waves that are mutually destructive, and the effective portion 

 of the surface is so small that the energy is reflected strictly 

 in accordance with the familiar law that the angles of incidence 

 and reflection are equal. But with long waves, such as we have 

 to deal with in sound, the effective part of the surface is not 

 small, and all of the surface with which we have to deal con- 

 tributes something to the sound ultimately heard. This is 

 well illustrated in the case of reflection of sound by the wall 

 of a house or the face of a cliff, or by the " sounding-board " 

 suspended over a pulpit, or by the difficulty of determining 

 whether a vehicle is coming from right or left along a street 

 when the observer is in a side-street. The degree to which the 

 problem is affected by the length of the waves, or, what is 

 essentially the same thing, the pitch of the note, may be well 

 illustrated by listening to the shrill ticking of a watch held in 

 the hand, and noticing that if a postcard or book is held a few 

 inches from the ear the sound of the ticking is no longer heard. 

 But if the lower pitched ticking of a pendulum clock is employed 

 the introduction of the book hardly reduces the loudness. 



In class B, which may be suggested by fig. 1, what little 

 efficiency these instruments possess, and for the most part this 

 is very little indeed, depends solely jipon reflection ; the source 



