ght acting
through the whole thickness of the film, which is a sine qua non to obtain
a clear ground, i.e., not stained blue.

To prepare by floating, pour the solution in a shallow tray, which needs
not to be more than 20x34 inches, 30 inches being the width of the drawing
paper usually employed; then roll the paper and place it on the solution.
Now, taking hold of it by two corners, draw it out slowly: the paper will
unroll by itself. This operation can be done by diffused daylight, but,
of course, the paper should be dried in a dark room. It dries rapidly.
Endless rolls are prepared by machinery. To expose, the drawing is placed
in the printing frame, face downwards, and the sensitive paper laid over
it. The whole is then pressed into contact by interposing a cushion
between the lid of the frame and the paper, and exposed so that the rays
of light fall _perpendicularly_ upon it.

The cyanofer preparation is quite sensitive. From half a minute to two
minutes exposure, according to the intensity of the light and the
thickness of the coating, is sufficient in sunshine to reproduce a drawing
made on the ordinary tracing paper. In the shade, by a clear sky, the
exposure is about five times longer, and varies from half an hour to an
hour and more in cloudy weather, but then the design is seldom perfectly
sharp.

The progresses of the impression is followed by opening one side of the
printing frame and examining the proof. The exposure is sufficient when
the paper is tinged brown on the parts corresponding to the ground of the
design. The image appears then negative, that is, yellowish on a tinged
ground.

Another and more safe method of ascertaining the correct time of exposure,
which can be employed concurrently with the other, is to place a few
strips of the same sheet of sensitive paper between the margin of the
design, upon which a few lines have been traced, and the paper, and,
without opening the frame, to draw one of them, from time to time, and dip
it in the developing solution. If the whole strip be tinted blue, the
proof is not sufficiently exposed; but if the lines soon appear with an
intense coloration on the yellowish ground of the paper, and the latter do
not turn blue in a minute, at the most, the exposure is right. By excess,
the lines are with difficulty developed or broken.

For developing, we provide with three wooden trays lined with lead or
gutta-percha, or, more economically, coated with