and its application to taking Portraits from the Life. 2 f 21 



justment before reading the suggestions contained in that ex- 

 cellent communication. The amount of shortening which 

 should be given to the camera, where the lens is fifteen inches 

 focus, does not commonly exceed three-tenths of an inch. 

 If the luminous focus be used, the proof comes out indistinct. 

 In the subsequent process of mercurializing, it is of little 

 importance what is the angular position. Several experi- 

 menters were for a time under the idea that an angle of 45 

 or 48 was a necessary inclination, in order that the plate 

 should take the vapour ; this arose from a misinterpretation 

 of the printed account. Plates mercurialize equally well in a 

 horizontal as in any other position ; perhaps a slight inclination 

 may be of advantage, in allowing the vapour to flow with uni- 

 formity over the iodized surface, but the chief use of an angle 

 of 45, is to allow the operator to inspect the process through 

 the glass. 



Sometimes it is advantageous to heat the mercury a second 

 time, when the proof is not distinctly evolved at first. Indeed, 

 it occasionally happens, that a proof which did not evolve at 

 all at first, will come out quite fairly on raising the tempera- 

 ture of the mercury again. 



M. Daguerre recommends two methods of removing the 

 sensitive coating from the plate, by washes of hyposulphite of 

 soda, and a solution of common salt. The former answers 

 perfectly, the second only indifferently well. There is, how- 

 ever, another process, which is very simple, and has an advan- 

 tage over the former of these in cheapness. It adds not a 

 little to the magic of the whole operation, in the eyes of those 

 who are unaccustomed to chemical results. The plate, having 

 been dipped into cold water, is placed in a solution of common 

 salt, of moderate strength ; it lies without being acted upon at 

 all ; but if it be now touched on one corner with a piece of 

 zinc, which has been scraped bright, the yellow coat of iodide 

 moves off like a wave and disappears. It is a very pretty 

 process. The zinc and silver forming together a voltaic 

 couple, with the salt water intervening, oxidation of the zinc 

 takes place, and the silver surface commences to evolve hydro- 

 gen gas ; whilst this is in a nascent condition it decomposes 

 the film of iodide of silver, giving rise to the production of 

 hydriodic acid, which is very soluble in water, and hence in- 

 stantly removed. 



This process, therefore, differs from that with hyposulphite. 

 The latter acts by dissolving the iodide of silver, the former 

 by decomposing it. It is necessary not to leave the zinc in 

 contact too long, or it deposits stains, and in large plates the 

 contact should be made at the four corners successively, to 

 avoid this accident. 



