y as many cross-bars, in order that by
opening one section, from time to time, the operator can follow the
progressive changes resulting from the action of light on the iron salts.
To print, the frame should be placed in the light in such a manner as the
luminous rays fall perpendicularly upon the drawing or cliche. The reason
of this is obvious, since the sensitive paper is not in direct contact
with the design, but separated by the material upon which it is drawn.

During the insolation--whose time depends necessarily from the more or less
transparency of the cliche, and, also, from the intensity of the
light(7)--the paper assumes first a violet tint, which gradually
intensifies to a dark shade; then this tint fades, becomes brownish, then
pale lilac, while the parts under the lines--that is, the design--upon which
the light has, therefore, no action, are visible by keeping the original
yellow-green tint of the prepared paper. It is when the lilac color is
produced that the exposure is sufficient.

To ascertain when the exposure is correct, a few black lines can be traced
on one of the edges of the margin of the design, and strips of the
sensitive paper placed upon them to serve as _tests_ in operating, as it
will be explained in the description of the Cyanofer process. When one of
them is taken out and show, by being washed in water, a clear white line
on a deep blue ground, the exposure is at an end. One understands that
the blue color of the ground is more or less intense according to time of
insolation, for the chemical actions between the reduced and the
non-reduced iron salts is so much more complete as the salts acted on are
more or less deoxidized, that is, reduced to ferrous salts; and that to
obtain the maximum of effect, which, therefore, depends on the allowable
time of exposure, the drawing ink should be opaque and non-actinic as far
as possible, because when, on testing, the lines are tinted the exposure
should be discontinued. However, a slight coloration of the lines is not
very objectionable, for it disappears by a longer washing after the
development.

The image is developed and fixed by washing in water two or three times
renewed. The water must be free from calcareous salts; these salts
converting the iron into carbonates which impart an ochrey tinge to the
proof. Rain water--any water in which no precipitate is thrown down by
the addition of a few drops of a weak solution of silver nitrate-