28 FRAGMENTS OF SCIENCE. 



II. 

 RADIATION. 1 



1. Visible and Invisible Radiation. 



"DETWEEN the mind of man and the outer world are 

 _D 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 

 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 assem- 

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

 selects and responds to those for the perception of which 

 it is specially 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. 



1 The Kede Lecture delivered in the Senate House before the 

 University of Cambridge, May 16, 1865. 





