224 ON LIGHT. 



which catch and reflect a small portion of it, as when in 

 a thick fog the bull's-eye of a lanthorn seems to thro\V 

 out a broad diverging luminous cone, consisting in 

 reality of the whole illuminated portion of the fog. The 

 moon is seen in virtue of the sun's light thrown upon it. 

 Where the moon is not we see nothing, though we are 

 very sure that when in the course of its revolution it 

 shall arrive in the place we are looking at, we shall see 

 it, and that if our eyes could be transferred to the moon's 

 place, wherever it may be in the firmament (if not 

 eclipsed), we should from it see the sun. There then, 

 at all times, is the light of the sun, but not visible as a 

 thing. It exists as an agency. What is true of the sun 

 is no doubt equally so of a star ; so that when we look 

 out on a dark night, though we are sure that all space is 

 continually being crossed in every direction by the lines 

 of its communication, along all which it is active; and in 

 particular, that all the dark space immediately around 

 us (outside of the earth's shadow) is, so to speak, flooded 

 with the sun's lights we yet perceive only darkness, 

 except where our line of vision encounters a star. 



(7.) What then is Light % or, in other words, what is 

 the nature of that communication by which not only 

 information is conveyed to our intellectual and per- 

 ceptive being ; but chemical and various other changes 

 are operated even on inorganic matter by processes 

 originating as it would seem in sources situate in the 

 most distant regions of space (for, be it observed, it has 

 been clearly proved that the light of the stars does pro- 

 duce photographic effects power/ul enough to imprint 



