o
admit air but no light; the plates will dry in from eight to twelve
hours. They are best prepared in the evening, and, if the closet is
good, will be dry in the morning.

After the plates are dry they may be packed face to face with nothing
between them, in a double-cover paper box, and put in a dark closet
free from sulphureted hydrogen gas, until ready for use. I have kept
plates for three months in this way, and they were in good condition.
Great care should be used in developing these plates, as they are
sensitive to the red; get used to developing in a dark part of the
dark room; occasionally you may look at the process of development in
a little stronger light.

The exposure through the yellow screen with an erythrosine plate is
about the same as if you had no orthochromatic plate--a plain plate
instead--provided you are not using too dark a yellow on your screen.
This can only be determined by experience. I will give to a common
plate about four seconds, an orthochromatic plate under the same
conditions five seconds.

The yellow glass screen is prepared as follows: Take a piece of best
plate glass--common cannot be used--clean it nicely; take another
large plate glass, or anything that is level and true, level it with a
small spirit-level. Now take the cleaned piece of glass and coat it
with

AURENTIA COLLODION.

Ether 5 oz.
Alcohol 5 oz.
Cotton 60 grs.

The aurentia to be added to suit your judgment; it takes a very small
quantity to make an intense yellowish-red collodion. Pour it on the
center of the glass, flow it to the edges, and before it sets place it
on the level glass and allow it to set; when set put it in a rack to
dry.

Should it dry in ridges, the collodion may be too thick, and it must
be thinned down with equal parts of alcohol and ether. A single piece
of plate glass, about one-eighth inch thick, coated with aurentia
collodion, is all that is required with an erythrosine plate. Or,
after a piece has been successfully coated, another piece of the same
plate glass, and the same size, may be cemented together with balsam,
having the coated aurentia side between the two glasses; the edges may
then be bound with paper.

In using different colored solutions, collodion, etc., I have found
that one will change the focus and the other not. With some screens
you must focus with them in their