II 



RADIATION 1 



1. Visible and Invisible Radiation 



BETWEEN the mind of man and the outer world are 

 interposed the nerves of the human body, which 

 translate, or enable the mind to translate, the im- 

 pressions of that world into facts of consciousness and 

 thought. 



Different nerves are suited to the perception of differ- 

 ent impressions. "VVe 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 x 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 phenom- 

 ena of radiation, and, notwithstanding its marvellous sen- 

 sibility to certain impressions of this class, it is singularly 

 obtuse to other impressions. 



1 The Rede Lecture delivered in the Senate House before the University of 

 Cambridge, May 16, 1865. 



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