[pgvizze] SCREEN IN THE PHOTO-MECHANICAL PROCESS 33 
When 7 is equal to one-half of J, the line of equal illumination coin- 
cides with the sides of the screen’s squares. 
The curves are plotted in Fig. 7 
2 
for values of +, increasing’ in arith- 
1 D 
metical progression from 5!, to 1. 
If we take for 7 that critical 
amount of illumination which is just 
sufficient to prevent the deposit of 
ink on the corresponding part of the 
print, the curves of Fig. 7 indicate 
the shape of the dots. They are 
black dots under the opaque square 
of the screen, and white dots under 
en 
the transparent square. For Rio 
they become alternate black and 
white squares, the print being an ex- 
act copy of the screen. We will call 
this the middle tone of the print. 
A cross-lined screen, in which 
the opaque and transparent lines are 
of equal width gives precisely the 
same curves as the chess-board 
screen. The screen and diaphragm 
must be adjusted to fit as shown in 

PRG 
Fig. 8; this condition is fulfilled when the distance, f, from the screem 
to the plate is : 

F 
Du 0 4 
Den © 
F being the distance from the plate to the 
diaphragm, n the number of the screen’s 
lines to the inch, and 4 the diagonal of the 
opening in the diaphragm. We will see 
later on that this is also the proper adjust- 
ment when the transparent and opaque 
lines of the screen are not of equal width. 
With this screen, we take D, Fig. 8, 
for origin of co-ordinates, DA and DC as 
axes, and we call 2a the distance from the 
centre of the transparent square to the 
centre of the intersection of the opaque lines. The equations of the 
curves of equal illumination are precisely the same as for the chess- 
sec. ITI., 1895. 3. 
