[pevize] SCREEN IN THE PHOTO-MECHANICAL PROCESS 41 
prints middle tone, The construction of the curves for any other dots 
shows that they are not connected. 
With a larger or a smaller diaphragm, FG and the other sides of 
the squares cease to be lines of equal illumination ; there are, therefore, 
no dots of that shape in the print, and its middle tone no longer consists 
of equal black and white squares. The construction of the curves shows 
that the larger dots are connected. Too small a diaphragm gives con- 

PKG Lie 
nected black dots; one too large gives connected white dots. Both are 
illustrated in Fig. 17 for diaphragms much out of adjustment. With 
better adjustment the effect would be less marked. 
Let us now take an original made with black ink on white paper. 
To be an exact copy the print must be formed of tones equivalent to 
those of the original. Where the pure white of the paper is preserved in 
the original, black dots must be absent in the print. Here we meet a 
difficulty : the dots in the engraved block are in relief and the depressions 
between them are very shallow. If more than a few dots are omitted, 
there is nothing left to support the inking-roller, which is pressed 
against the bottom of the depressions and the print is not clean. Conse- 
quently, unless the extent of the high lights be very small, we cannot 
produce a print which will be an exact copy of the original; the high 
lights, instead of being pure white, must be a tone consisting of black 
dots, which may, however, be as fine as it is possible to print. We will 
consider the case where the extent of the high lights is so small that the 
dots can be omitted. : 
Let Z be the intensity of the light coming from the pure whites of 
the original: leaving out the screen, the illumination received by the 
photographic plate at a point corresponding to the pure white is CL, C 
being a constant depending upon the length of exposure, aperture of 
diaphragm, etc, The intensity for any other tone of the original may be 
represented by pL, p being a fraction smaller than one; it causes an 
illumination of the plate equal to CpL. Now insert the screen; the 
