ON RADIANT HEAT: 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 clue 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 
wavs of rendering those sonorous vibrations notoniy 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 
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 
