CONTEMPORARY ADVANCES IN PHYSICS 283 



which observe the bodies which oscillate. But if no one had ever felt 

 or seen the quiverings of the humming fork, the ringing bell, or the 

 resounding drumhead, we should be handicapped severely for dis- 

 covering the true nature of sound. 



Precisely so handicapped are we for discovering the nature of 

 light. It may be that light is the vibration of a substance; but if so, 

 the eye does not perceive that substance nor anything which fluctuates ; 

 it translates the vibration into a constant sensation. Moreover, 

 we have no other sense which perceives that substance. When the 

 filament of a lamp is incandescent, nothing is observed to pulsate on 

 its surface; nothing is observed to go up and down or back and forth 

 in the surrounding vacuum. Our instruments also fail to detect any- 

 thing of which the vibrations are light. One may measure light with 

 a photographic film or a bolometer; but the undulations — if such there 

 be — are translated, in the one case into a steady rate of chemical 

 change, in the other into a steady flow of electric current. In short, the 

 eye and all our instruments register light as the ear registers tone, and 

 not at all as the eye or the hand may register the quiverings of a sound- 

 ing body; and therefore they do not report that light is vibratory. 

 And if it be true that tangible matter is itself of the nature of a wave- 

 motion, then the sense of touch must respond to these waves as the 

 eye to light or the ear to sound, not reporting anything vibratory and 

 not perceiving any medium which vibrates, but translating the vibra- 

 tions into a constant sensation. 



Therefore, to test whether light or matter or electricity is a wave- 

 motion, one must make such experiments as could be made to test 

 whether sound is a wave-motion, if there were no instrument able to 

 perceive the vibrations of matter except the ear. Let us then suppose 

 ourselves required to prove, to someone unable or unwilling to use any 

 instrument except the ear, that sound is of the nature of waves ; and 

 consider how we should go about it. 



The ear, we are told, is able to make distinctions of "loudness," 

 "timbre," and "pitch." The two latter, interesting as they are, are 

 of no immediate concern in this enterprise. It is sufficient to know 

 that the ear makes distinctions in loudness, which according to the 

 wave-theory correspond to distinctions in amplitude of vibration. 

 Again, we have no concern with the exact relation between amplitude 

 and loudness. What matters is that the former controls the latter, 

 and therefore the latter reveals the former. Though the ear cannot 

 detect the cyclic variations of the density and pressure of the air which 

 make the sound, it can detect fluctuations in the amplitude of these 

 cyclic variations. To put this statement into briefer language of 



