ON RA DIANT HEA T. 55 



insensible. The sensible processes give direction to the 

 line of thought; but this once given, the length of the 

 line is not limited by the boundaries of the senses. Indeed, 

 the domain of the senses, in Nature, is almost infinitely 

 small in comparison with the vast region accessible to 

 thought which lies beyond them. From a few observations 

 of a comet, when it comes within the range of his telescope, 

 an astronomer can calculate its path in regions which no 

 telescope can reach: and in like manner, by means of data 

 furnished in the narrow world of the senses, we make our- 

 selves at home in other and wider worlds, which are trav- 

 ersed by the intellect alone. 



From the earliest ages the questions, " What is light ? " 

 and " What is heat? " have occurred to the minds of men; 

 but these questions never would have been answered had 

 they not been preceded by the question, " What is sound? " 

 Amid the grosser phenomena of acoustics the mind was 

 first disciplined, conceptions being thus obtained from 

 direct observation, which were afterward applied to phe- 

 nomena 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, molds the air 

 around it into undulations 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 notonly tangible 

 but visible; and it was not until numberless experiments 

 of this kind had been executed that the scientific investi- 

 gator 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 pro- 

 duced by an agitation of the auditory nerve the thought 

 eoon 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 



