The sensitized paper and the
negative are placed between the pad and the plate glass, and the whole
is squeezed together by pressure applied at the periphery of the glass
and of the back-board. Both the glass and the back-board spring under
the pressure, and it results that the sensitized paper is not so
severely pressed against the negative near the center of the glass as it
is near the edges. If at any point the sensitized paper is not pressed
hard up against the negative, a bluish tinge will appear where a white
line or surface was expected. With an efficient printing frame and
suitable negatives, these blue lines will never appear, and it was to
prevent the production of defective work that I undertook to improve the
pad of the printing frame.
_The Printing Frame Used in Ordinary Photography._--Very naturally, I
first examined the printing frame used in ordinary photography. This
frame is extremely simple, and is very well adapted to its use. It is,
undoubtedly, the best frame for blue process printing, when the area of
the glass is not too large. The glass is set in an ordinary wooden
frame, while the back-board is stiff and divided into two parts. A flat,
bow-shaped spring is attached by a pivot to the center of each half of
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.
Pages:
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75