[peviLe] SCREEN IN THE PHOTO-MECHANICAL PROCESS 59 
Fig. 26 is for copying from originals through a chess-board screen ; 
it requires two and a half times the exposure now given to produce the 
same grain. 
Fig. 27 is also for copying from an original through a chess-board 
screen : it has multiple apertures and therefore requires shorter exposure. 
All the above are for copying from originals and are given merely 
as illustrations of this paper. To obtain correct results, some of the 
following shapes have to be employed. 
Ngee 
244m 


IRIEL, DASE 1.29): 
Fig. 28 is the single star aperture for the chess-board screen. 
Fig. 29 is the double star aperture for a cross-lined screen in which 
the lines are of equal width and parallel to the sides of the screen. 
Fig. 30 is a five-aperture diaphragm for the chess-board screen only. 
The general rule for making a multiple diaphragm for the chess-board 
screen is that each star must be inscribed in the alternate squares of a 
chess board. The rule for the cross-lined screen is that there must be 
one pair or several pairs of apertures; the line joining the centres of the 
components of any one pair must be inclined at 45° to the lines of the 
screen. 
Fig. 31 is a single aperture and double exposure diaphragm for the 
chess-board screen ; it is made in the shape of a reversible Waterhouse 
diaphragm. The first exposure is given through the full aperture, after 
which the cap is put on the lens, the diaphragm reversed end for end in 
the slot and the second exposure given. This diaphragm may, like the 
others, be made with multiple apertures by the rules given above. 
