r, by means of a
plate of clear glass, laid over both and clamped to the board.

4. Expose the whole--in a clear sunlight--from 4 to 6 minutes. In a
winter's sun, from 6 to 10 minutes. In a clear sky, from 20 to 30
minutes.

5. Remove the prepared paper and pour clear water on it for one or two
minutes, saturating it thoroughly, and hang up to dry.

The sensitive paper may be readily prepared, the only requisite quality
in the _paper_ itself being its ability to stand washing.

Cover the surface evenly with the following solution, using such a brush
as is generally employed for the letter-press: 1 part soluble citrate of
iron (or citrate of iron and ammonia), 1 part red prussiate of potash,
and dissolve in 10 parts of water.

The solution must be kept carefully protected from light, and better
results are obtained by not mixing the ingredients until immediately
required. After being coated with the solution, the paper must be laid
away to dry in a dark place, and must be shielded entirely from light
until used. When dry, the paper is of a yellow and bronze color. After
exposure the surface becomes darker, with the lines of the tracing still
darker. Upon washing, the characteristic blue tint appears, with the
lines of the tracing in vivid contrast. Excellent results have been
obtained from glass negatives by this process.--_Proc. Eng. Club,
Phila._

* * * * *

REPRODUCTION OF DRAWINGS IN BLUE LINES ON WHITE GROUND.

A.H. HAIG.

The following process for making photographic copies of drawings in blue
lines on white background was invented by H. Pellet, and is based on the
property of perchloride of iron of being converted into protochloride on
exposure to light. Prussiate of potash when brought into contact with
the perchloride of iron immediately turns the latter blue, but it does
not affect the protochloride.

A bath is first prepared consisting of ten parts perchloride of iron,
five parts oxalic or some other vegetable acid, and one hundred parts
water. Should the paper to be used not be sufficiently sized, dextrine,
gelatine, isinglass, or some similar substance must be added to the
solution. The paper is sensitized by dipping in this solution and then
dried in the dark, and may be kept for some length of time. To take a
copy of a drawing made on cloth or transparent paper, it is laid on a
sheet of the sensitive paper, and exposed to light in a printing frame
or unde