[pevizze] SCREEN IN THE PHOTO-MECHANICAL PROCESS Si 
between the second and the first exposure and modifying the shape of 
the aperture accordingly, the only limit being the length of exposure 
which the operator is willing or can afford to give. It is unnecessary to 
add that the order of the exposures is im- 
material; they may commence with the 
cross and end with the whole figure. 
The cross exposure may be made less 
than the one with the whole figure, but 
only up to a certain point, The point is 
reached when the quantity under the radi- 
cal in (7) becomes equal to zero. When 
the calculations are made, it is found that 
the figure assumes the shape of a star, like 
Fig. 13. The whole figure and the inner 
cross being now identical, there is no 
longer any necessity for two separate ex- 
posures. The value of 6, which is the side of the square formed by join- 
ing the four inner angles of the star, is: 
CIE TY 
vib Co b = 0:363 Ap 
INT TT 
A being$the diagonal of the star. The 
height of the triangles forming the 
four wines is: 
DS 
p= 338 4; 

t 
<- 
nette 
The dots produced by this aperture 
are given in Fig. 14. The smaller ones 
still are circles, the larger ones being 
bound by arcs of circles, ellipses and 
hyperbolas. Their areas are : 


qd Area. q | Area. 

0°05 0:05 | 0°30 | 0-2836 
0:10 | 0°10 | 0-35 | 03235 
0‘15 0:15 | 040 | 03689 
0-20 01990 | 0:45 | 0°4215 




0°25 02437 0:50 | 0.50 

Fig. 14. 
The three smaller dots, which are 
circles, are correct: The error for the 
other dots, although larger than with the two separate exposures. is small. 
and I believe that this shape of aperture will answer all practical purposes. 
