SILVER PRINTS. 



369 



chloride (calomel) is also formed as a deposit; by treating the 

 plate again with a solution of sodium sulphite, ammonia, or 

 ferrous oxalate, a further change is produced whereby the silver 

 chloride is either dissolved out or again reduced to metallic silver, 

 whilst the calomel is converted into minute particles of metallic 

 mercury or other mercurial compound; the end result in every 

 case being that the total deposit is thicker, or at least more opaque, 

 than at first, the more so where most silver originally existed, so 

 that the lights and shadows are intensified relatively to one 

 another just as with the wet collodion plate, but owing to different 

 chemical and physical actions. Ferricyanide of potassium in 

 conjunction with uranium or lead salts furnishes another set of 

 intensifying agents acting in a somewhat similar fashion. 



Expt. 392. To Prepare Silver Prints. If a sheet of suitably 

 prepared paper, a film of sensitised collodion, or any similar layer 

 of material sensitive to photographic action is exposed to light 

 with some opaque or semi-transparent object lying on its surface, 

 wherever the light is not prevented from reaching the paper by 

 the intervening object chemical action will take place, so that the 

 paper is blackened or otherwise altered ; where, however, the paper 

 is covered with an entirely opaque portion of the object to be 

 copied, the access of light is prevented, so that the paper under- 

 neath remains unchanged ; 

 whilst, similarly, a semi- 

 transparent part of the object 

 will allow more or less light 

 to pass, giving a tint inter- 

 mediate between that of the 

 unaltered paper and that 

 which has been exposed to 

 the full power of the light. 



Fig. 200 represents the 

 reproduction in this way of 

 a skeleton leaf. A piece of 

 ordinary note paper or of 

 unglazed white paper is 

 soaked in a solution of com- 

 mon salt (1 to 1J ounce salt 

 to 1 pint water) for a few 

 minutes, and then taken out 

 and hung up to dry, conveni- 

 ently on a string stretched 

 across a room after the fashion of a laundress's clothes line ; or, 

 instead of soaking the paper, it is floated on the surface of the 



2A 



Fig. 200. Skeleton Leaf (Silver Nega- 

 tive Print). 



