48 ROYAL SOCIETY OF CANADA 
2nd. Cover the original with a sheet of white paper, remove the 
plate-holder to the dark room, insert the screen, and give a second 
exposure, equal to the first one, on the sheet of white paper, using the 
star diaphragm of correct size (Fig. 13). If this exposure were given 
alone, and the plate fully developed and intensified, the negative would 
show an extremely fine opaque dot under the centre of each transparent 
square of the screen. After receiving the two exposures, the negative is 
developed and the usual operations follow. 
Using the same notation as before, the first exposure has caused an 
illumination, at any particular point of the plate, equal to 
pL. 
The second exposure gives the illumination, 
qb, 
so that the total illumination at the point of the plate under consideration is 
pL + ql. 
The outlines of the dots are the curves for which 
pL + ql =i, 
or 
i 
PR = 7 
L is the illumination produced, during the first exposure, by the highest 
lights of the original ; it is also the illumination under the centre of the 
transparent squares during the second exposure. In both cases we have 
so timed the exposures that Z is just sufficient to produce printing density 
on the negative. But that is precisely the definition of 7; therefore 
DR 
and the equation of the dots becomes : 
D'AMIENS 
This equation means that the tone, pL, of the original is represented 
by dots, from the outlines of which the fraction 1 — p of the diaphragm 
is visible. We have seen, in the first part of this paper, that the areas of 
these dots are practically correct. 
The first exposure may be given with any kind of diaphragm, and 
must be just sufficient to impress on the plate the highest lights of the 
original, whether on white paper or not. 
The second exposure can be given on any uniform source of illumin- 
ation, provided it is so timed as to just produce printing density under 
the centre of the transparent squares of the screen, If white paper be 
used, it is preferable to place it out of focus, so that the grain of the 
