[ppvize] SCREEN IN THE PHOTO-MECHANICAL PROCESS 53 
Assuming, as before, that the highest light, Z, of the subject, for 
which the density of the transparency is D,, must be represented by pure 
white in the print, we must have 
D+ d= M, 
d, being the greatest density of the screen. Combining with (14), there 
comes 
D—D,=4,—d. 
Replacing both members of this equation by their values from (12) and 
(13), we obtain : 
— à log. p = — fi log. ¢. 
This equation is satisfied by: 
a = f, 
DIT: (15) 
The first relation shows that the development factors of the trans- 
parency and screen must be equal ; a strongly developed transparency 
requires a strongly developed screen. 
The second relation means that the outline of the dot by which the 
tone pl of the subject is represented, is the line of the screen which 
received the illumination pl while the screen was being made. In order 
to produce a correct print, the surface inclosed by this line must be pro- 
portional to p for the white dots, and to (1 — p) for the black dots. We 
know that this is approximately the result of an exposure behind a chess- 
board screen through a star or double exposure diaphragm ; we are, 
therefore, able to make vignetted screens possessing the gradations re- 
quired to produce exact copies of anoriginal. For this purpose we expose 
a photographic plate on a piece of white paper placed out of focus, the 
diaphragm and screen being adjusted as explained in the first part of this 
paper. 
The theory for copies in the camera is so much like the theory for 
contact copies in the printing frame, that further explanations are not 
needed. 
The vignetted screen having to be placed in contact with the photo- 
graphic plate, its face cannot be protected by a glass cover ; it can only 
be varnished. Very likely it will soon be scratched or damaged, and will 
have to be replaced frequently. The operator should have a set of screens 
of various densities to select from; light screens for light transparencies, 
and dark screens for intense transparencies. There does not appear to be 
any reason why every operator should not make his own vignetted 
screens ; they are produced by operations with which he is familiar, and 
which do not present any special difficulty. The plate on which the 
screen is to be made is, we have seen, exposed behind a chess-board screen 
through a star or double exposure diaphragm. ‘The correct exposure is 
and 
