RADIANT HEAT AND ITS RELATIONS. 75 



of acoustics the mind was first disciplined, conceptions 

 being thus obtained from direct observation, which 

 were afterwards 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, moulds the air around it into un- 

 dulations or waves, which speed away on all sides with 

 a certain measured velocity, impinge upon the drum of 

 the ear, shake the auditory nerve, and awake in the 

 brain the sensation of sound. When sufficiently near 

 a sounding body we can feel the vibrations of the air. 

 A deaf man, for example, 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 rendering those sonorous vibrations 

 not only tangible but visible; and it was not until num- 

 berless experiments of this kind had been executed, 

 that the scientific investigator abandoned himself 

 wholly, and without a shadow of misgiving, 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 any- 

 thing imparted to it. But if light be produced by an 

 agitation of the retina, what is it that produces the 

 agitation? Newton, you know, supposed minute par- 

 ticles to be shot through the humours 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 



