1HE MAGIC WAND IN PHOTOGRAPHY 263 



By some power not understood by man, the sun is able to 

 form new substances. In the dark, chlorine and hydrogen 

 are simply chlorine and hydrogen ; in the sunlight they com- 

 bine as if by magic into a totally different substance. By the 

 same unexplained power, the sun frequently does just the op- 

 posite work; instead of combining two substances to make 

 one new product, the sun may separate or break down some 

 particular substance into its various elements. 



For example, when the sunlight falls upon silver chloride a 

 chemical action immediately begins. Some chlorine gas escapes. 

 Some silver is also liberated, but it reacts with the, as yet, un- 

 affected silver chloride and gives it a red or purple color. In 

 time the entire mass of silver chloride is affected. 



The magic wand in photography. Let us coat one side of a 

 glass plate with silver chloride suspended in gelatine. We must 

 be very careful to do this in a dark room, 1 otherwise the sun- 

 light will act upon the silver chloride before we are ready. Lay 

 a horseshoe on the plate and carry it into the sunlight for a 

 minute. The light causes some of the chlorine to escape, but 

 the silver remains with the rest of the silver chloride, darkening 

 it. All of the plate was affected by the sun except the portion 

 protected by the horseshoe, which does not allow light to reach 

 the plate. If, now, the plate is carried back to the dark room and 

 the horseshoe is removed, we see on the plate an image of the 

 horseshoe. But we must not take this image into the light, be- 

 cause the silver chloride which was protected by the horseshoe 

 is still present and would be strongly affected by the least light, 

 and would spoil the image. 



But a photograph on glass, which must be carefully shielded 



from the light, would be neither pleasurable nor practical. If 



there were some way by which the unaffected silver chloride 



could be totally removed, we could take the plate into any 



1 That is, a room from which ordinary daylight is excluded. 



