[pgvizce] SCREEN IN THE PHOTO-MECHANICAL PROCESS AS 
As p decreases, g increases, slowly at first and more rapidly after, 
until the value of 1 is reached for p = 0:14, This value of g, represent- 
ing the full effective aperture of the diaphragm, it follows that the illu- 
mination is insufficient to produce white dots for tones of the original less 
bright than 0:14 Z; they are represented by solid black in the print. 
The smaller black dots are circles; their equation is : 
‘) 1 
_ 9 
(a — ay + @— yt =2e(-—1). 
q EL 
The larger dots are bound by composite curves ; their equations are 
somewhat complicated, but it is always possible to construct the curves 
of any diaphragm by points. The diaphragm is placed in a number of 
ae ee eee ee ee ee ee 

Bros: Fig. 19. 
different positions over the screen, and its visible area measured for each 
position. The curves of equal illumination are then interpolated between 
the points so determined. 
It is at once seen how imperfect the print is. The area of the white 
dots should be proportional to p and the area of the black dots to 1—p; 
that is, they should be like the dots of Figs. 12 and 14. Instead of this, 
we see that the original tone, 0°5 Z, which is one-half the brightness of 
the white paper, is represented in the print by the black dot shown by 
