LIGHT. 115 



though we may render observations more convenient ; may 

 by its means fix images seen on rare and favourable occasions, 

 and may preserve permanent and infallible records of the 

 past state of astronomical objects. 



The effect of light on chemical compounds affords us a 

 striking instance of the extent to which a force, ever active, 

 may be ignored through successive ages of philosophy. If 

 we suppose the walls of a large room covered with photo- 

 graphic apparatus, the small amount of light reflected from 

 the face of a person situated in its centre would simulta- 

 neously imprint his portrait on a multitude of recipient sur- 

 faces. "Were the cameras absent, but the room coated with 

 photographic paper, a change would equally take place in 

 every portion of it, though not a reproduction of form and 

 figure. As other substances not commonly called photo- 

 graphic are known to be affected by light, the list of which 

 might be indefinitely extended, it becomes a curious object of 

 contemplation to consider how far light is daily operating 

 changes in ponderable matter how far a force, for a long 

 time recognised only in its visual effects, may be constantly 

 producing changes in the earth and atmosphere, in addition 

 to the changes it produces in organised structures which are 

 now beginning to be extensively studied. Thus every portion 

 of light may be supposed to write its own history by a change 

 more or less permanent in ponderable matter. 



The late Mr. George Stephenson had a favourite idea, 

 which would now be recognised as more philosophical than it 

 was in his day, viz. that the light, which we nightly obtain 

 from coal or other fuel, was a reproduction of that which had 

 at one time been absorbed by vegetable structures from the 

 sun. The conviction that the transient gleam leaves its per- 

 manent impress on the world's history, also leads the mind 

 to ponder over the many possible agencies of which we of the 

 present day may be as ignorant as the ancients were of the 

 chemical action of light. 



