Let me try to answer this. Before exposure the daguerreotype plate is coated
evenly with silver halides. When light hits the plate the silver halides are
reduced to metallic silver in the highlights. Ordinary gel/silver developer
would simply hasten this process. In the shadows, silver halides would be
covering the metallic silver plate. During fixing all the silver hallides
would be removed thus you would have an image formed out of metallic silver
surrounded by metallic silver. Ergo, no image visable.
What I believe happens in mercury development is that the mercury condenses
on the plate in the region where the silver halogens have been reduced
by the action of light to metallic silver where they amalgamate(sp?) with the
metallic silver. This does not happen in the shadow areas because they are
covered with AgI and AgBr at the time of development. After development
the remaining silver hallides are converted into water soluable compounds by
the sodium thiosulfate and washed away revealing the silver surface which
appears dark while the little mercury drops appear light.
It is the optics that reverses the image. You are looking at the original.
In modern positive/negative photography the image is reversed again during
printing. Remember, you print with the "shiney side" up which is not the
emulsion side.
Bob Schramm
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