VIII. 

 RADIATION. 



1. Visible and Invisible Radiation. 



Between the mind of man and the outer world are in- 

 terposed 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 marvellous sensibility to 

 certain 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 incom- 

 petent to evoke its power, -while others never reach it at 

 all, being absorbed by the humors of the eye. To all rays 



