positions; take away the screen, and
the picture appears out of focus. I cannot fully explain why it is,
and for this reason will not make the attempt; experience alone can
teach it.

Another thing that has been tried lately is to do away with the yellow
screen by substituting a yellow coating direct on the plate. No doubt
the focus on an object that requires absolute sharpness is somewhat
affected by the use of a glass. We have been successful, on a small
scale, to coat the plate with the following yellow solution:

Place in a tray enough of a saturated solution of tropaeolin in wood
alcohol to cover the plate; allow it to remain ten seconds. It is
necessary that the plate should be bathed previously in erythrosine
and dried. Before applying the tropaeolin, which, being in alcohol,
dries in a few minutes, have some blotting paper on hand, as the
solution gathers in a pool and leaves bad marks on the end of the
plate.

The plate can be developed in the usual way. Try it and see the
results.--_Reported in the Beacon._

* * * * *

PLATINOTYPE PRINTING.[1]

[Footnote 1: A communication to the North London Photographic
Society.]

Platinotype, which may be considered to be the most artistic of
photographic printing processes, may be separated into its three
modifications--the hot bath and cold bath, in which a faintly visible
image is developed, and the Pizzighelli printing-out paper. The hot
bath process, again, may be divided into the black and white and sepia
papers. I intend to give you a rough outline of the preparation of the
paper and working of these modifications, concluding by demonstrating
the hot bath method, and handing around prints by it.

Platinotype may almost be styled an iron printing process, for, while
no trace of iron or its salts is found in the finished print, certain
salts of iron are mixed with the platinum salt, which is platinum
combined with two atoms of chlorine (PtCl2), as a means for readily
reducing it; this, however, cannot be effected without the presence of
neutral oxalate of potash, hence the use of the oxalate bath. There is
no platinum in the paper for the cold bath process, it being coated
with ferric oxalate mixed with a very small quantity of chloride of
mercury--somewhere about one grain to an ounce of ferric oxalate
solution. When dry it is ready for exposure, which is about three
times less than with silver printing.

It is