[DEVILLE] SCREEN IN THE PHOTO-MECHANICAL PROCESS 57 
The difference x of the serial numbers of the sizes 4’ and J is governed 
by the relation : 
a 
100 
(1-05) — =) 
n 
100 
"= (1-05)2. 
or 
The classification of screens given above was calculated by this 
formula. 
The principle of this series is easy to understand. The idea is to have 
two numbers on each diaphragm, one, a number of inches, for adjusting 
it to the lens, and another one, the serial number, for adjusting it to the 
screen. 
A full set of diaphragms for one lens and one screen consists of 
fourteen numbers ; they are taken from the general series and fitted to 
the lens. For more than one screen, there must be as many additional 
stops as between the coarsest and the finest screen. Thus for screens of 
91 and 110 lines, the set consists of 14 + 2 + 2, or18 diaphragms, because 
91 lines is two stops coarse and 110 lines two stops fine. 
Although the basis of this system is a distance of the screen of 4 
inch, the series may be used for any other distance, but with this differ- 
ence, that the numbers on the diaphragms are for another screen than 
100 lines. For instance with a distance of the screen of } inch, the num- 
bers correspond to a screen of 83 lines, and it is from it that the other 
screens have to be counted coarse or fine. 
The illustrations represent the actual sizes of numbers of the series. 
Those who are familiar with the photo-mechanical process will find them 
somewhat smaller than the square stop to which they are accustomed, and 
may believe that they are proportionately slower. It is not so; their 
rapidity must not be estimated from their comparative sizes. There are 
two reasons for this. The first one has already been stated ; as now used, 
there is always a portion of the square aperture hidden by the screen, 
while the full aperture of the new stops is, in most cases, effective. The 
second reason is that to obtain the same fineness of grain as at present, 
the new stops require a coarser screen which means a larger aperture. 
Where a longer exposure is required, it is, as in copying from negatives, 
by reason of the process and not on account of the sizes of the apertures, 
The exposure can be reduced by the use of multiple apertures instead 
of single ones ; this can be done with any kind of screen. The exposure 
is reduced as many times as there are apertures in the diaphragm. It 
must be borne in mind that multiple apertures require great perfection 
in the screen and very accurate adjustments. For a double aperture, the 
screen must be three times more perfect and the adjustments three times 
