56: ROYAL SOCIETY OF CANADA 
progression from this size, the ratio being 1:05. Each diaphragm bears 
two numbers: the first one is the serial number and indicates the place of 
the diaphragm in the series ; the second one is a number of inches which re- 
presents the extension of the camera to which it is adapted, when the screen 
used is ruled 100 lines to the inch or made from this ruling, and also when 
the distance from the screen to the plate is one-sixth of an inch. The 
camera extension is readily ascertained by nailing a yard measure on the 
tailboard and adjusting it to show the distance from the diaphragm to 
the plate. So, an operator using a screen of one hundred lines to the 
inch has only to select the diaphragm bearing the number of inches 
which, after focussing, he reads on the tailboard of the camera. 
If the screen should not be ruled 100 lines to the inch, then, for 105 
lines take the next smaller diaphragm of the series; for 110 lines, take 
two sizes smaller; for 116 lines, take three sizes smaller and so on. When 
the screen is 95 lines, take the next size larger; if 91 lines, take two sizes 
larger, etc. The classification of the screens is as follows :— 
71 lines to the inch is 7 stops coarse. 
75 & rE 6 
(cma are f 5 
82 qu F ‘ : 4 
86 ‘ : 5 3 i 
91 ‘ à 2 
100 as s 0 
105 à 1 
1:10 rs 2 
116 : 3 
122 ae + 
128 . : : 5 rs 
134 . $ 6 
141 | 7 
The number of inches to be marked on each diaphragm is given by 
equation (16): 
F 200 A 
— VOL. . 
For the exceptional square shape this number of inches is deduced 
from equation (4), and is only one-half of the above. 
A screen ruled n lines to the inch requires a diaphragm in which the 
diagonal 4’ of the single aperture is such that : 
Al’. 00 
ZAR 
