[pgvizze] SCREEN IN THE PHOTO-MECHANICAL PROCESS 39 
parent squares, for improving the shadows, and under the intersection of 
the opaque lines, for improving the lights. This is done for the lights 
by making the transparent lines of the screen wider than the opaque 
lines. Fig. 15 shows a screen in which the transparent lines are double 






























































































































































































































































Fra. 15. 
the width of the opaque lines; the perspective of the square diaphragm 
is adjusted over the intersection of the opaque lines, AC being equal to 
PO. With the screen made of lines of equal width, the diaphragm, in 
this position, was entirely hidden by the opaque lines : now, there are four 
triangular portions visible from the plate which give under O a certain 
amount of supplementary illumination, the effect of which is to improve 
the high lights. The shadows, however, are injured in proportion as the 
lights are improved. 
I have now to explain why the diaphragm must be adjusted as in 
Fig. 15 ; this necessitates a digression. 
The grain which we have been considering so far is one consisting of 
isolated black and white dots; only in the middle tone do the dots come 
into contact, and then only at one point, the corner of the squares. It 
must not be supposed that any screen or any diaphragm produces a grain 
of this kind ; on the contrary, it is the exception and not the rule. It is 
not obtained with other screens than the cross-lined and chess-board, and 
