140 MODIFICATIONS IN THE DAGUERREOTYPE. 



adjustment before reading the suggestions contained in that excellent communication. 

 The amount of shortening which should be given to the camera, where the lens is fif- 

 teen inches focus, does not commonly exceed three tenths of an inch. If the luminous 

 focus be used, the proof comes out indistinct. 



540. In the subsequent process of mercurializing, it is of little importance what is 

 the angular position. Several experimenters 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 mercurial- 

 ize 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 uniformity over the iodized 

 surface, but the chief use of an angle of 45 is to allow the operator to inspect the pro- 

 cess through the glass. 



541. 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. 



542. 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, however, another 

 process, which is very simple, and has an advantage over the former of these in cheap- 

 ness. 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 be- 

 ing 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 oflT like a wave and 

 disappears. It is a very pretty process. The zinc and silver forming together a vol- 

 taic couple, with the salt water intervening, oxidation of the zinc takes place, and the 

 silver surface commences to evolve hydrogen gas ; while 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 instantly removed. 



543. 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, aqd in large plates the contact 

 should be made at the four corners successively, to avoid this accident. 



544. After the proof is washed, all the defects in the preparation of the plate become 

 apparent. If a film of mercury has existed on it, due to its not having been burned 

 sufficiently long, there will be found a want of distinctness in the shadows ; or if the 

 plate has not been burned at all, perhaps the former impressions which have been 

 obtained will reappear. This accident frequently happened in my earlier trials, when 

 care had not been taken to give a due exposure each time to the spirit flame. Spectral 

 appearances of former objects, on different parts of it, emerged an interior with Paul 

 Pry coming out, when the camera had been pointed at a church. 



545. There is no difficulty in procuring impressions of the moon by the Daguerreo- 



