the back-board. The two halves of the back-board are hinged together by
ordinary butts. Four lugs are fastened to the back of the frame, and,
when the back-board is placed in position, the springs may be swung
around, parallel to the line of the hinges, and pressed under the lugs,
so that the back of the back-board is pressed most severely at the
center of each half, while the glass is prevented from springing away
from the back-board by the resistance of the frame at its edges. Unless
the frame is remarkably stiff, it will resist the springing of the glass
more perfectly in the neighborhood of the lugs than elsewhere. It will
now be seen that, on account of the manner in which the pressure is
applied, the back-board tends to become convex toward the glass, while
the adjacent surface of the glass tends to become concave toward the
back-board; and that with such a frame, the pressure upon all parts of
the sensitized paper is more nearly uniform than when the pressure is
applied in the manner before described. With a small frame of this
description, a piece of ordinary cotton flannel is used between the
back-board and the sensitized paper, and, with larger sizes, one or more
thicknesses of elastic woolen blanket are substituted for the cotton
flannel. There is an advantage in having a hinged back-board like that
which has been described, because, when the operator thinks that the
exposure to sunlight has been sufficiently prolonged, he can turn down
either half of the back and examine the sensitized paper, to see if the
process has been carried far enough. If it has not, the back-board can
be replaced, and the exposure continued, without any displacement of the
sensitized paper with respect to the negative. This is an important
advantage.

_An Efficient Blue Process Frame, for Printing from Large Negatives, or
for Printing Simultaneously from many Small Ones._--In order to be
efficient, such a frame must be capable of keeping the sensitized paper