Page 104 - The Wellington photographic handbook

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As before, N represents the
negative, L the lens, and B the
bromide paper, S being a shutter
actuated by a knob outside the
apparatus, by which the exposure
is effected. In its simplest form
there is a lid at B, and the whole
apparatus is taken into the dark-
room, a piece of sensitive paper
is put inside it at B, by means
of the lid, which is then fastened,
and the apparatus is carried out
into the open air, so that there is
an unlimited view of the sky
above the negative N. The ex-
posure is then made by means of
the shutter S. Direct sunlight
must, of course, be avoided.
In a fixed focus enlarger of
this sort, there is no need to
trouble about stop values, as the
apparatus always works at the same stop.

Using the same brand of paper, and the same (or a similar)
negative, the exposure will always be in proportion to the strength
of the light as measured by a meter which can be placed on the top
of the apparatus just before exposing. Once the correct exposure
with a given negative is known and the time taken by the meter
slip to darken has been recorded, all future exposures for the same
or a similar negative can be worked out in the following manner :

Multiply the number of seconds of the known (i.e., first) ex-
posure, by the number of seconds taken by the meter slip to darken
under the new conditions, and divide by the number of seconds
it took to darken when the known (i.e., first) exposure was made.
The answer will be the exposure required in seconds.
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