[pevicze] SCREEN IN THE PHOTO-MECHANICAL PROCESS 31 
that the perspective of the diaphragm fits as shown in ABCD, Fig. 2, 
the diagonals of the perspective being equal and parallel to the, sides of 
the squares of the screen. 
Let J be the illumination when the whole of the diaphragm is visible, 
as in Fig. 2, and designate by 2a the side of a square of the screen; the 
area of the diaphragm’s perspective is: 
2a". 
Now displace the perspective by moving the point of sight on the 
plate, and take O as origin of the co-ordinates, O4 and OY as the axes. 
The centre of the perspective comes in P, Fig. 3, its co-ordinates being x 
? 5S ) [=] 
>4 


LL 
LL 
GY, YY, 
ty Yy 4 
Yj 

and y. The whole surface of the diaphragm has ceased to be visible : 
two corners are cut off by the opaque squares of the screen. The area of 
one corner is: 
(a a x)", 
and the area of the other : 
Ce) 
So the visible portion of the diaphragm is: 
DE (a — 2)? — (a — y}, 
and the illumination : 
= D] 
2a 
from which we obtain: 
(a — 2) + (a—y)' = 2a (1—). (1) 
