The number of copies which can be taken from a single original
photographic picture, appears to be almost unlimited, provided that every
portion of iodine has been removed from the picture before the copies are
made. For if any of it is left, the picture will not bear repeated
copying, but gradually fades away. This arises from the chemical fact,
that solar light and a minute portion of iodine, acting together (though
neither of them separately), are able to decompose the oxide of silver,
and to form a colourless iodide of the metal. But supposing this accident
to have been guarded against, a very great number of copies can be
obtained in succession, so long as great care is taken of the original
picture. But being only on paper, it is exposed to various accidents; and
should it be casually torn or defaced, of course no more copies can be
made. A mischance of this kind having occurred to two plates in our
earliest number after many copies had been taken from them, it became
necessary to replace them by others; and accordingly the Camera was once
more directed to the original objects themselves, and new photographic
pictures obtained from them, as a source of supply for future copies. But
the circumstances of light and shade and time of day, &c. not altogether
corresponding to what they were on a former occasion, a slightly different
but not a worse result attended the experiment. From these remarks,
however, the difference which exists will be easily accounted for.