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History and Practice of the Art of Photography by Henry Hunt Snelling
page 37 of 134 (27%)
Beyond this a protecting influence is powerfully exerted;
and notwithstanding the action of the dispersed light,
which is very evident over the plate, a line is left,
perfectly free from mercurial vapor, and which, consequently,
when viewed by a side light, appears quite dark.
The green rays are represented by a line of a corresponding tint,
considerably less in size than the luminous green rays.
The yellow rays appear to be without action, or to act negatively,
the space upon which they fall being protected from the
mercurial vapor; and it consequently is seen as a dark band.
A white line of vapor marks the place of the orange rays.
The red rays effect the sensitive surface in a peculiar manner;
and we have the mercurial vapor, assuming a molecular arrangement
which gives to it a fine rose hue; this tint is surrounded by a
line of white vapor, shaded at the lowest extremity with a very
soft green. Over the space occupied by the extreme red rays,
a protecting influence is again exerted; the space is retained
free from mercurial vapor and the band is found to surround
the whole of the least refrangible rays, and to unite itself
with the band which surrounds the rays of greatest refrangibility.
This band is not equally well defined throughout its whole extent.
It is most evident from the extreme red to the green;
it fades in passing through the blue, and increases again,
as it leaves the indigo, until beyond the invisible chemical
rays it is nearly as strong as it is at the calorific end
of the spectrum.

Images on Daguerreotype plates which have been completely obliterated
by rubbing may be restored, by placing it in a tolerably strong solution
of iodine in water.
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