HOCUS, AND OETICS 37 
make the image not more than one-fourth of the size of the 
object ; then make a negative of the foot rule, and, calling the 
distance between the rule and sensitive plate D and the ratio 
of reduction r, which can be found by measuring the length 
of the image on the negative and dividing into the length of 
the foot rule, then f = D X r+(r-+1)’*. Example: a rule 
of 150mm was found to give an image of 13.4mm, D was 
800 mm, then rv = 150 + 13.4 = 11.19; then f = 800 x 11.19 
— (11.19 -+ 1)? = 67.78 mm. 
Another Method.—Focus on a near object, then move the 
object further away from the lens by a distance D, refocus, 
note the distance through which the focussing screen has been 
moved, and call this a2. Again move the object away from the 
lens another distance equal to the first distance D, and 
call the distance the focussing screen has moved 0, then © 
f=V2DXbXa(b+a)~+ (a—b). The lens must re- 
main stationary in this method. Let D=96,a—=2,b=—¥; 
Pei Vo DON 2) 2 2-2) (2) 
V 480 + 1.5 = 14.61. 
Another Method.—Reverse the lens in its flange, focus on 
infinity, or a very distant object, and call this distance b. 
Replace the lens in its ordinary position, and again focus on 
infinity. Now focus on a near object at a distance D, measure 
the extension of the camera beyond the infinity mark, and 
call this y; then f= V(D—b)y. Lett D=48, b=8, 
y= ¥,; then f= V (48— 8) 4 = 4.472. 
Pinhole Method—The focus can also be found by means 
of a pinhole. Focus on infinity with the lens, and measure 
the size of the image or make a negative. Then place a 
pinhole in the position of the lens, and produce an image of 
the distant object exactly the same size; the distance of the 
pinhole from the image is the focus. A simple thin lens may 
be used instead of the pinhole. Or, if a lens of known focus 
