[pgvizze] SCREEN IN THE PHOTO-MECHANICAL PROCESS 51 
The easiest way of making the transparency is to employ one of the 
enlarging and reducing cameras which have a partition in the middle, 
carrying the lens, and one of the ends arranged to receive the negative. 
The ordinary copying board is covered with white paper and set some 
distance away from the negative ; it is lighted, as usual, by electric light 
or otherwise. With a negative of proper density, the exposure need not 
be excessive, although longer than with the process as now worked. 
It is possible to employ an arrangement like the enlarging lantern, 
provided the source of light be of the right kind. It must be a surface 
of uniform illumination; the lime-light or the arc-light will not do. 
Incandescent gas-light may answer the purpose. After focussing, and 
before exposing the plate, the condenser should be carefully adjusted. 
This may be done by marking, with ink or pencil, on a piece of white 
cardboard, the outline of the diaphragm’s aperture and inserting it in the 
diaphragm’s slot. The condenser and light are now moved until the 
image of the latter is formed on the cardboard ; it must fully cover the 
diaphragm’s aperture and be perfectly uniform. If it were otherwise, the 
shape of the dots would be governed, not by the diaphragm, but by the 
shape of the light. 
LV. Copying THROUGH VIGNETTED SCREENS. 
The screens which we have been investigating consist of opaque and 
transparent parts; a vignetted screen is semi-opaque over its whole 
surface, and divided into minute zones of varying degrees of opacity. 
This screen must be placed in contact with the photographic plate ; its 
use is therefore restricted to dry plates. 
It may be made to copy from an original or transparency, or from a 
negative ; from a practical point of view, the first case is the only one to 
be considered. : 
In copying from the original, the screen is placed as usual in the 
camera, but in contact with the plate ; the shape of the diaphragm is im- 
material, and so is its size, provided it is not too large. A transparency 
may be copied in the camera or in a printing frame. The arrangement 
of the camerashould be something like that described for copying from 
negatives, with the exception that any illuminant is suitable; the are 
light would answer the purpose admirably. For the printing frame, the 
transparency is stripped and placed between the plate and the screen. 
Exposure must not be given to diffused light, but to light emanating from 
a point, like an are light; the frame must be kept in the same position 
during the whole exposure. The theory is identical for the camera and 
for the printing frame ; we will investigate the latter only. 
In a perfect transparency, the opacities are inversely proportional to 
the light intensities which they represent. Let Z be the greatest light 
