In each
of my large frames I use the commercial plate glass; instead of the
cushion of cotton flannel, or of flannel, I use a cushion filled with
air of sufficiently high pressure to flatten all cockles, and to press
all parts of the sensitized paper closely against the negative; and
instead of the hinged back-board I use a back-board made in one piece
and clamped to the frame of the glass at its edges. Connected with the
cushion is a pressure gauge, and a tube with a cock, for charging the
cushion with air from the lungs. Experience shows what pressure is
necessary with any given paper, and the gauge enables one to know that
the pressure is neither deficient nor in excess of that which is safe
for the glass.
[Illustration: PLAN. COTTON FLANNEL REMOVED.]
[Illustration: SECTION AT CO.]
_The Construction of the Air-Cushion._--The expense of such an
air-cushion seemed at first likely to prevent its being used; but a
method of construction suggested itself, the expense of which proved to
be very slight. The wooden back-board, as constructed, is made in one
piece containing no wide cracks. It has laid upon it some thick brown
Manila paper, the upper surface of which has been previously shellacked
to make it entirely air-tight. Upon this shellacked surface is laid a
single thickness of thin paper of any kind; even newspaper will answer.
Its object is simply to prevent the sheet rubber, which forms the top of
the air-cushion, from sticking to the shellacked paper.
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