222 ON LIGHT. 



idea of at least the general nature of the mechanism by 

 which it seems now agreed, with hardly a dissentient 

 voice, that the peculiar communication between distant 

 objects which we call light is effected; and by which, or 

 by some mechanism of a nature still more recondite, and 

 at present perhaps beyond our conception of possibility, 

 it must be so. 



(4.) That we see, is proof of a communication of some 

 sort between the eye and the thing seen. That we can- 

 not see in the dark, is proof that such communication is 

 not the mere act of the eye. And that one object is 

 capable of impressing a photographic picture of itself on 

 another, is proof that the eye, though essential to seeing^ 

 has nothing whatever to do with the process by which 

 sucli communication is performed. And furthermore, 

 the immense variety and extent of the chemical agencies 

 of light as displayed in its action both on organic and 

 on inorganic matter, revealed to us by the late discov- 

 eries in photography, assign to it a rank among natural 

 agents of the highest and most universal character; and 

 have even rendered it exceedingly probable, if they have 

 not actually demonstrated, that vision itself is nothing 

 but the mental perception of a chemical change wrought 

 by its action on the material tissue of the retina of the 

 eye. 



(5.) At all events, it is not by any sympathy, or abso- 

 hite direct relation between the eye and the object, that 

 the latter is seen. The intermediate space, and indeed 

 all sj>ace, is concerned in the process. An object is not 

 seen unless it be in a certain state, which we call " lumin- 



