PHOTOGRAPHY, SCIENTIFIC ASPECTS 



tory, where he used his various chemicals. Field opera- 

 tions, therefore, were seldom attempted. But, in 1854, 

 Spiller and Crookes published in the Philosophical 

 Magazine the account of a method of keeping plates 

 moist for several days that made field operations pos- 

 sible. The basis of this process was the use of the salts 

 of zinc which have the property of imbibing moisture 

 from the atmosphere. By incorporating a certain 

 quantity of these salts in the moist film of the plate, 

 drying was prevented for several days. So that by 

 using a somewhat large and clumsy set of plate-holders, 

 specially made for the purpose, the photographer could 

 make extensive excursions; and field photography 

 soon became popular. 



COLLODION-EMULSION PROCESS 



After the discovery of this last phase of plate-making 

 little progress was made in photography for a decade. 

 Then, in 1864, Bolton and Sayce invented their "col- 

 lodion-emulsion process," and a new impetus was 

 given to the art. It will be recalled that in the processes 

 for coating the plates, used heretofore, it was necessary 

 to coat the plate with the collodion mixture of the bro- 

 mides, dry it, and then dip it in a solution of silver 

 nitrate. In the new "emulsion" process the emulsion 

 contained all the chemicals for the necessary reaction, 

 so that the plate could be made simply by pouring the 

 emulsion over the surface and allowing it to set. But, 

 when first introduced, this process was found to have 

 a serious defect the plates frequently "fogged" in 



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