Photograph courtesy of The 35mm Handbook by Michael Freeman ISBN 0 906286 21 2
Modern photography is based on the combination of two fundamental processes. The production of an image of a subject by means of a lens and the provision of a sensitive material which can be altered permanently by the action of light.
Photography is possible because of the fact, known to the chemists of the 18th century, that most chemical compounds of silver darken on the exposure to light.
A favorite demonstration was to impregnate some paper or fine leather with silver nitrate and by laying on these sensitized materials leaves and other objects and then exposing them to light. The exposed material would darken and when the leaves and other objects were removed an outline or silhouette of the object was visible. But even while viewing this image it would also darken as there was no way to stop the unexposed material from darkening. In other words there was no way to Fix the image.
This darkening of the material is the action of light on the silver salt turning it into metallic silver.

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