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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