Photograph courtesy of Cameras From Daguerreotypes to Instant Pictures by Brian Coe
In 1837 Loius Jaques Mandre Daguerre developed a process based on the exposure of a highly polished silver surface to Iodine vapor which produced on it a layer of light sensitive silver iodide.
 An exposure of twenty minutes or more in the camera obscura produced no visible change on the plate but by then exposing it to the vapor from heated mercury an image was developed by the amalgamation of the mercury vapor with the tiny specs of silver produced by the light action. The plate was fixed by washing it in a very strong solution of common salt (Sodium Chloride.)
Daugerre announced his discovery on the 7th of January 1839 and the working details were published in August 1839.
This "Daguerreotype" process was a positive only process and was a once only process.

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