[pgvizze] SCREEN IN THE PHOTO-MECHANICAL PROCESS 55 
The cross-lined screen, employed in this way, gives twice as many dots 
as otherwise ; a coarser ruling must be taken to obtain the same fineness: 
of grain. If, for instance, it should be desired to have the grain of 150: 
lines to the inch, the screen for the double aperture should be ruled 106: 
lines to the inch. It must also be observed that, with the double aper- 
ture, the squares of the middle tone are not turned around 45°; to: 
keep them turned in the print as they now are, the ruling must be 
parallel to the sides of the screen, not to the diagonals. The double 
aperture diaphragm, illustrated in Fig. 29, is for screens so ruled. 
The adjustment of the chess-board screen is the same as for the 
double aperture, and is given by equation (16), n being the ruling from 
which the screen is made and = inch the side of a square of the screen. 
In the aperture of Fig. 21, 4 is the diagonal of the inner star. 
In equation (16) we have two variable quantities, J and f; the 
adjustment can therefore be made by changing the size of the dia- 
phragm, or by changing the distance of the screen, or by both. So long 
F 
as the product of J and f is equal to n° it does not matter whether the 
diaphragm is small and the screen far from the plate, or the diaphragm 
large and the screen close to the plate; the result is identical. In prac- 
tice, there are many reasons why the diaphragm, and not the screen, 
should be made adjustable ; I will only give one. It is essential that the 
distance f from the screen to the plate be uniform throughout, and, being 
so very small, no adjustable arrangement can accomplish that satisfac- 
torily. 
The screen should be perfectly flat, and, for large sizes at least, 
nothing but plate glass should be used for the negative or transparency. 
The screen and plate should be separated by blocks of silver or glass, 
planed down to an accurate thickness, and they should be brought by 
pressure into close contact. This pressure must be applied to the edges 
and not to the centre of the plate. 
I consider + of an inch as being about the right distance between the 
plate and the screen. This allows a glass cover 7's inch thick to protect 
the screen and leaves about 75 inch between the face of the plate and the 
glass cover, which is quite enough. It is this, distance which I have 
adopted for the following classification, 
All diaphragms, whether consisting of a single aperture, or of a pair 
of apertures, or of multiple apertures, and whatever their shapes may be, 
are included in one general series. In number zero of the series, the dia- 
gonal of the single aperture is one-quarter of an inch. There is an excep- 
tion for the square diaphragm now in general use with cross-lined screens : 
for this shape or multiples of this shape, the diagonal of number zero is 
one-half inch. No smaller sizes are likely to be required for this process. 
The linear dimensions of the other diaphragms increase in geometrical 
