[pevizce] SCREEN IN THE PHOTO-MECHANICAL PROCESS 47 
For improving the shadows a supplementary exposure is given with 
the diaphragm #F, the diagonal of which is equal to one-half of CD. 
This exposure is from ten to fifty times the prelisainary exposure ; the 
longer it is, the greater is the range of tones reproduced by the print. It 
has no effect on the lights. Making it eighteen times the preliminary 
exposure gives approximately the dots of Fig. 22. The middle tone of 
the print represents the tone, 0:25 JL, of the original ; the shadows less 
bright than 0-14 Z come out as solid black. In other words, the range 
of tones reproduced is from + Z to L. Designating by k the supplement- 
ary exposure, the range of the print is from 
il 
Ont: 
ET 
4 
For instance, a supplementary exposure equal to thirty times the pre- 
liminary one gives a range from 1 to 1. 
We can now understand better the mode of action of the diaphragms 
of Figs. 21 and 27. In both, the equivalent of the preliminary exposure 
is given through the rhombus-shape extensions at the corners. The sup- 
plementary exposure in Fig. 27 is given through the four smaller squares : 
in Fig. 21 it results from the star-shape of the main aperture. The pre- 
liminary and main exposures, in the method described above, may be re- 
placed by a single exposure through the central part of the diaphragm of 
Fig. 27. 
. The gradations may be improved by substituting for the original a 
sheet of white paper during part of the supplementary exposure. Let 
us assume that the original is on white paper and that the highest lights 
are represented by the pure white. After the preliminary and main 
exposures let us give a supplementary exposure of 4 on the original, then 
substitute the sheet of white paper and give an exposure of 2. The 
range of tones reproduced by the print is from + Z to Z, as in the 
example cited above, but the middle tone now represents the tone 
033 L of the original, instead of 0:25 Z as formerly. That is a very 
great improvement. On the other hand, the total exposure which, by 
the first method, was 21:5 times the preliminary exposure, is reduced to 
9:5, less than one-half of what it was before, 
The methods which we have described are all more or less imper- 
fect. Theoretically the correct method, which involves two separate 
exposures, is as follows: 
Ist. Expose, without the screen, on the original, which we will 
assume again to be on white paper, and time the exposure so that if the 
plate were then fully developed and intensified, it would just show a trace 
of the highest lights of the original, The size and shape of the diaphragm 
are, of course, immaterial, but for the sake of simplicity in the explan- 
ations, we use the same diaphragm as in the second exposure. 
