LIGHT. 99 



obliged to be contented with a loss of intensity in the telescopic 

 image. 



Now, let us suppose that the minutest details in the 

 image of an object seen in a given telescope, and with a given 

 power, are noted ; that then a photographic plate is placed in 

 the focus of the same telescope so as to obtain a permanent 

 impression of the image which has been viewed by the eye- 

 glass. Could the observer, by throwing a beam of condensed 

 light upon the photograph, enable himself to bring out fresh 

 details ? or, in other words, could he use with advantage a 

 higher power applied to the illuminated photograph than that 

 which he used for direct vision ? 



It is, perhaps, hardly safe to answer a priori this question ; 

 but the experiment of reproducing photographs would seem 

 to show that more detail than that produced by the initial 

 light cannot be obtained, and that we cannot expect to increase 

 telescopic power by photography, though we may render 

 observations more convenient, may by its means fix images 

 seen on rare and favourable occasions, and may preserve per- 

 manent and infallible records of the state of astronomical 

 objects at the periods of observation. 



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 photogra- 

 phic apparatus, the small amount of light reflected from the 

 face of a person situated in its centre would simultaneously 

 imprint his portrait; on a multitude of recipient surfaces. 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 photographic are known to 

 be affected by light, the list of which might be indefinitely 

 extended, it becomes a curious object of contemplation to con- 

 sider 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 exten- 



H 2 



