RADIANT HEAT AND ITS RELATIONS 81 



observation, whicli were afterward applied to phenomena of a 

 character far too subtle to be observed directly. Sound we 

 know to be due to vibratory motion. A vibrating tuning- 

 fork, for example, moids the air around it into undula- 

 tions or waves, which speed away on all sides with a cer- 

 tain measured velocity, impinge upon the drum of the ear, 

 shake the auditory nerve, and awake in the brain the sen- 

 sation of sound. When sufficiently near a sounding body 

 we can feel the vibrations of the air. A deaf man, for ex- 

 ample, plunging his hand into a bell when it is sounded, 

 feels through the common nerves of his body those tremors 

 which, when imparted to the nerves of healthy ears, are 

 translated into sound. There are various ways of render- 

 ing those sonorous vibrations not only tangible, but visi- 

 ble ; and it was not until numberless experiments of this 

 kind had been executed that the scientific investigator 

 abandoned himself wholly, and without a shadow of mis- 

 giving, to the conviction that what is sound within us is, 

 outside of us, a motion of the air. 



But once having established this fact — once having 

 proved, beyond all doubt, that the sensation of sound 

 is produced by an agitation of the auditory nerve — the 

 thought soon suggested itself that light might be due to 

 an agitation of the optic nerve. This was a great step in 

 advance of that ancient notion which regarded light as 

 something emitted by the eye, and not as anything im- 

 parted to it. But if light be produced by an agitation of 

 the retina, what is it that produces the agitation ? New- 

 ton, you know, supposed minute particles to be shot 

 tnrough the humors of the eye against the retina, which 

 he supposed to hang like a target at the back of the eye. 

 The impact of these particles against the target, Newton 



