20 FRAGMENTS OF SCIENCE. 
energy. In no other sense has the convertibility of force 
any scientific meaning. 
Grave errors have been entertained as to what is really 
intended to be conserved by the doctrine of conservation. 
This exposition I hope will tend to remove them. 
CHAPTER II. 
RADIATION.* 
1. Visible and Invisible Radiation. 
BETWEEN the mind of man and the outer world are inter- 
posed the nerves of the human body, which translate, or 
enable the mind to translate, the impressions of that world 
into facts of consciousness and thought. 
Different nerves are suited to the perception of different 
impressions. We do not see with the ear, nor hear with 
the eye, nor are we rendered sensible of sound by the 
nerves of the tongue. Out of the general assemblage of 
physical actions, each nerve, or group of nerves, selects 
and ' responds to those for the perception of which it is 
specially organized. 
The optic nerve passes from the brain to the back of the 
eyeball and there spreads out, to form the retina, a web of 
nerve filaments, on which the images of external objects 
are projected by the optical portion of the eye. This nerve 
is limited to the apprehension of the phenomena of radia- 
tion, and, notwithstanding its marvelous sensibility to cer- 
tain impressions of this class, it is singularly obtuse to 
other impressions. 
Nor does the optic nerve embrace the entire range even 
of radiation. Some rays, when they reach it, are incompe- 
tent to evoke its power, while others never reach it at all, 
being absorbed by the humors of the eye. To all rays 
which, whether they reach the retina or not, fail to excite 
vision, we give the name of invisible or obscure rays. All 
non-luminous bodies emit such rays. There is no body in 
nature absolutely cold, and every body not absolutely cold 
emits rays of heat. But to render radiant heat fit to affect 
* The Rede Lecture delivered in the Senate House before the Uni- 
versity of Cambridge, May 16, 1865. 
