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 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.
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