ioo CORRELATION OF PHYSICAL FORCES. 



sively studied. 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. In 

 one sense we now know this to be true ; the rays of the sun 

 enable the leaves of plants to decompose carbonic acid gas, 

 absorbing and assimilating the carbon, and allowing the 

 oxygen to escape. Mr. Balfour Stewart supposes that the 

 want of photographic power in rays reflected from the leaves 

 of growing plants is due to the energy of the rays having 

 been used up in decomposing the carbonic acid. The carbon 

 thus segregated by the sun's rays is ready to give out heat 

 and light, whenever it may be recombined with oxygen by 

 combustion. The conviction that the transient gleam leaves 

 its permanent 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. 



I have used the term light, and affected by light, in speak- 

 ing of photographic effects ; but, though the phenomena 

 derived their name from light, it has been doubted by many 

 competent investigators whether the phenomena of photo- 

 graphy are not mainly dependent upon a separate agent 

 accompanying light, rather than upon light itself. It is, 

 indeed, difficult not to believe that a picture, taken in the 

 focus. of a camera-obscura, and which represents to the eye all 

 the gradations of light and shade shown by the original 

 luminous image, is not an effect of light ; certain it is, how- 

 ever, that the different coloured rays exercise different actions 

 upon various chemical compounds, and that the effects on 

 many, perhaps on all of them, are not proportionate in inten- 

 sity to the effects upon the visual organs. Those effects, 

 however, appear to be more of degree than of specific differ- 

 ence ; and, without pronouncing myself positively upon the 

 question, hitherto so little examined, I think we may, at all 



