214 FRAGMENTS OF SCIENCE. 



tuning-fork, for example, moulds the air round 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 ren- 

 dering 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 uncer- 

 tainty, 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 nerve of the ear, the 

 thought soon suggested itself that light might be due to 

 an agitation of the nerve of the eye. 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 optic nerve or retina, what is it that produces the 

 agitation ? Newton, you know, supposed minute particles 

 to be shot through the humors of the eye against the 

 retina, which hangs like a target at the back of the eye. 

 The impact of these particles against the target, Newton 

 believed to be the cause of light. But Newton's notion 

 has not held its ground, being entirely driven from the 

 field by the more wonderful and far more philosophical 

 notion that light, like sound, is a product of wave-motion. 



The domain in which this motion of light is carried on 

 lies entirely beyond the reach of our senses. The waves 



