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. 



KADIATION.* 



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. 



