II. 



RADIATION* 



1. Visible and Invisible Radiation. 



T3ETWEEN the mind of man and the outer world 

 T*-' are interposed 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 dif- 

 ferent 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 assem- 

 blage of physical actions, each nerve, or group of nerves, 

 selects and responds to those for the perception of 

 which it is especially organised. 



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 ex- 

 ternal objects are projected by the optical portion of 

 the eye. This nerve is limited to the apprehension of 

 the phenomena of radiation, 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 



* The Rede Lecture delivered in the Senate House before the 

 University of Cambridge, May 16, 1865. 

 28 



