Photocharging of thin films of silver iodide and its relevance to the Daguerre photographic process

Abstract

In the Daguerre photographic process^1–3, a polished silver-coated plate is exposed to iodine vapour, thereby forming a very thin film of silver iodide, usually about 30-nm thick⁴. After exposure, the image is developed by mercury vapour, which condenses preferentially where light has fallen, amalgamating with silver liberated from the silver iodide (and possibly with the substrate). The remaining silver iodide is dissolved with an aqueous photographic fixing solution (sodium thiosulphate). When viewed at most angles, a positive image of the scene is seen as areas of amalagam (diffusely reflecting), which appear white, on areas of polished silver (specularly reflecting), which appear black. The sharpness with which edges were reproduced in several old daguerreotypes led us to suspect that electrostatic processes might be involved, and we now report two experiments which confirm that.