EXPOSURE 75 
f: 100 as unity, so that the larger stops were marked with the 
relative luminosity, which gave as a fraction the relative 
exposures; for instance, f:6.3 was 256, and the exposure 
compared to that for f:100 was 1/256. All these systems 
have practically fallen into disuse in England and America, 
the ratio system alone being used. 
Exposure Meters.—The Watkins, Wynne, Imperial, and 
Beck meters are based on the measurement of the actinic 
power of the light by the darkening of a sensitive paper to a 
standard tint. Scales bearing the f: x value of the diaphragm 
and speed of the plate are brought into coincidence, and the 
exposure is then read off against the time taken by the paper 
to darken. | 
These meters should be exposed to the light falling upon 
the shadiest part of the subject in which full detail is re- 
quired. If there is no important shadow, test the direct sun- 
light. If there is any important shadow, face the meter to the 
sky and not to the sun, or use the shadow of the body. 
To judge the matching of the tint, hold the meter at arm’s 
length and observe through half-closed eyes, and note the 
time when the paper is neither lighter nor darker than the 
tint. An absolute match is not required. 
MAKING PAPER FoR ExposurE METERS.—In order to make 
a paper suitable for these meters, bromide paper should be 
immersed in the darkroom for 5 minutes in a 2.5 to 5 per 
cent solution of potassium metabisulphite or sodium nitrite, 
rinsed and dried. The tint caused by the action of light varies 
with the paper ; but a test should be made by burning 2 grains 
of magnesium ribbon, coiled into a spiral, at a distance of 
434 inches. The tint thus obtained will correspond to the 
correct exposure for a plate of Watkins’ speed 1 exposed at 
f: 8, midday in June sunlight. The colour should be matched 
with water colour. 
