FOCUS AND OERICS 49 
of 1/60 inch, the numbers being called Watkins-Power, or 
W. P. Nos. 
Nearest Nearest 
Decimals vulgar needle Working 
W.P.Nos. of inch fractions size distance 
1 .160 1/7 Sah EON HR saa 
Z O802 1/13 Bere bible cea 39 
3 053 1/19 1 40 in 
4 040 1/25 4 20 in 
5 032 IW 5 14 in 
6 027 1/38 i 10 in 
7 023 1/44 8 8 in. 
8 020 W/ 52 10 5 in. 
Multiply the W. P. No. of the aperture by the working 
distance from the plate. Use the result as the f number with 
which to calculate the exposure by table or meter. Whatever 
the calculated result is in seconds or fractions of a second, 
expose that same number of minutes or fractions of a minute. 
To find the best diameter of pinhole for a given extension 
of camera: divide the square root of the extension by 120. 
Example: extension = 11 in., square root of 11 = 3.316, and 
this divided by 120 — 0.0276 in. or approximately 1/36 in. 
To find the extension of camera for a given size hole, 
multiply the diameter of the hole by 120, and square the 
Recley) xataples diameter — 1/40) then 1/40)>< 120) — 3; 
and 3X 3 =9 in. 
CoNJUGATE Foc1.—The relative distances between the neg- 
ative and lens, and lens and image, for enlarging and reduc- 
ing, that is, copying in the camera or making lantern slides, 
can be found by the following simple rule: divide the longer 
side of the enlargement desired by the longer side of the 
negative, and the result will be the “times” of enlargement ; 
it should be noted that this is linear and not area enlargement. 
