206 Dr Fyfe on Daguerreotype. 



is with this view that I have been induced to investigate it ; and 

 I trust that the results I have obtained are not devoid of in- 

 terest, not only as tending to illustrate what I conceive to be 

 the action that occurs, but also as leading to some important 

 practical applications in the arts. 



On the first announcement of the mode of conducting da- 

 guerreotype, M. Arago conjectured that the effect, ultimately 

 brought out by the hyposulphite, might be owing to its wash- 

 ing off the iodine, and probably to the sulpluu' of the salt act- 

 ing on the silver, so as to convert it into a black sulphuret 

 which, contrasting with the white mercurial coating, gives 

 the dark and light parts of the pictm'e. It is evident, however, 

 that this is not the true rationale, because it is stated by 

 Arago himself, that a similar result is obtained by washing 

 the plate with solution of sea-salt, and where no sulphuret 

 can be produced. The action, in my opinion, is altogether 

 different ; it does not depend on the formation of compounds 

 differing in their natm'e and appearance, by the agency of the 

 hyposulphite, but on metallic sm'faces being presented, by 

 which the light is reflected differently. 



The first part of the process — the polishing — must be con- 

 sidered as merely mechanical ; and certainly the more high- 

 ly resplendent the surface, the more likely we are to suc- 

 ceed in getting a distinct image on it. With regard to the 

 use of the diluted acid, its action is not so easily accounted 

 for. That it should act on the silver, and form a nitrate, as 

 some have supposed, is not likely, when we consider its great 

 state of dilution, and the very minute quantity employed — 

 two or three drops being sufficient for a large plate. Accord- 

 ing to some it may wash off any copper adhering to the sur- 

 face of the silver. To ascertain whether or not it acted on 

 the metals, I raised a wall of wax-luting around a polished 

 plate, and kept on it diluted acid (1 to 16) for some time, rub- 

 bing the plate occasionally with cotton. On testing the fluid, 

 I could not detect the slightest trace of silver in it. Another 

 plate was acidified and heated as recommended by Daguerre. 

 It was then washed with distilled water ; and the washings 

 were tested for silver, with the same result. 



The fluid from the plate in the preceding experiment, when 



