THE MAGIC WAND IN PHOTOGRAPHY 127 



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, 

 if the sun's rays fall upon silver chloride, a chemical action im- 

 mediately begins, and as a result we have two separate sub- 

 stances, chlorine and silver. The sunlight separates silver 

 chloride into its constituents, silver and chlorine. 



.121. The Magic Wand in Photography. Suppose we coat 

 one side of a glass plate with silver chloride, just as we might 

 put a coat of varnish on a chair. We must be very careful 

 to coat the plate in the dark room,* otherwise the sunlight will 

 separate the silver chloride and spoil our plan. Then lay a 

 horseshoe on the plate for good luck, and carry the plate out 

 into the light for a second. The light will separate the silver 

 chloride into chlorine and silver, the latter of which will re- 

 main on the plate as a thin film. All of the plate was affected 

 by the sun except the portion protected by the horseshoe which, 

 because it is opaque, would not allow light to pass through 

 and reach the plate. If now the plate is carried back to the 

 dark room and the horseshoe is removed, one would expect to 

 see on the plate an impression of the horseshoe, because the 

 portion protected by the horseshoe would be covered by 

 silver chloride and the exposed unprotected portion would be 

 covered by metallic silver. But we are much disappointed 

 because the plate, when examined ever so carefully, shows 

 not the slightest change in appearance. The change is there, 

 but the unaided eye cannot detect the change. Some chemi- 



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



