[pevize] SCREEN IN THE PHOTO-MECHANICAL PROCESS 35 
manner as we have already done, and as the matter is quite simple, it is 
unnecessary to say anything further about it. 
Reverting to Fig. 7, it has been explained that the curves are lines of 
; 
equal illumination, the value of > changing by ,;, from one curve to the 
next one. Under the centre of the transparent square the illumination 
is Z; at the first curve or circle it is reduced to 12 of J; at the next 
curve to 18 of J, and so on till the last circle under the opaque square, 
where it is , of 7. Under the centre of this square it becomes zero. 
Thus each curve is the limit of the space or surface which has received 
an illumination not less than the fraction 
iS 
Il 
hy] = 
of the original intensity. 
It will be shown further on that for copying from negatives and for 
making vignetted screens, this space or surface must be proportional to g. 
The table underneath of the areas of Fig, 7 shows that this condition 
is not fulfilled by the square diaphragm and the screens employed ; the 
small dots, black and white, are too small, and the larger ones, near the 
middle tone, change too rapidly. 










q Area. q Area. q Area. q Area. 
0°05 0:0393 0°30 0°2373 0°55 05986 0°80 0°8480 
0°10 0°0785 0°35 0°2826 0°60 0°6648 0°85 0°8812 
0°15 071188 0°40 0°3352 0°65 0°7174 0°90 0°9215 
0°20 0°1570 0°45 0°4014 0°70 0°7627 0°95 0°9607 
0°25 071963 0°50 0°5000 0°75 0°8037 1:00 | 1:0000 


It will be observed that errors are equal for dots of same size, 
whether black or white ; in other 
tables we will only give the areas 
of the white dots. 
The circular dots can, with 
the chess-board screen, be made 
perfectly exact by a diaphragm 
of proper shape, but the discrep- 
ancy near the middle tone can- 
not be removed entirely, although 
it may be reduced as much as de- 
sired. 

The general shape of dia. 
phragm indicated by theory is eV - 
one involving two separate ex- Fra. 10. 
posures, and is shown in Fig. 10. It consists, for the first exposure, of 
a square of which the corners are cut off and replaced by triangles. 
