EADIANT HEAT AND ITS RELATIONS. 75 



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- 

 d illations 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 band 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 numberless 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 

 particles 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 against the target, Newton believed to be 



