CHAPTER XIV 



HEARING 



THE ear, like the eye, records amplitude of vibration ; loudness. 

 It also records rapidity of vibration, musical pitch, which corre- 

 sponds with colour. But it seems to have a more difficult 

 task than the eye, since it has to analyse, or at any rate has to 

 transmit information regarding the form of compound vibra- 

 tions. The meanings of these distinctions may be illustrated by 

 reference to a tracing on the cylinder of a phonograph. A 

 needle attached to the posterior surface of the thin metal plate 

 against which one speaks scratches the surface of a rotating 

 cylinder of hardened wax. Examined with a lens, the record is 

 seen to be an irregularly changing line. The depth of the marks 

 is a measure of loudness. Their varying number in a given 

 time indicates the changing pitch of the voice which produced 

 them. Their form is a record of the quality of its tone. The 

 work of the ear, so far as it consists in the estimation of the 

 amplitude and rapidity of pulsations of sound, is easy to 

 describe, but the acoustics of form are complicated. 



Light is transmitted as vibrations of aether. They are 

 transverse to the direction in which the light is travelling. 

 Sound cannot travel through a vacuum, since it is dependent 

 upon displacements of material particles. The particles move 

 forwards and backwards in the direction in which sound is 

 progressing. Sound is a sequence of pulsations, alternate con- 

 densations and rarefactions of the media which conduct it. 

 Their particles are first pressed together, and then rebound to 

 positions farther apart. A sequence of to-and-fro movements, 

 each smoothly continuous throughout the whole duration of a 

 pulsation, would produce a pure musical tone. Tuning-forks 

 carefully bowed settle down after a few seconds into unbroken 

 oscillations, which convey to the air the to-and-fro movements 

 of pure tones. Such tones vary in nothing but loudness and 

 pitch. If their pulsations are slow, we speak of the pitch as 

 " low "; if they are rapid, we say that their pitch is high. 



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