ith the graded portions of the negative
instead of with only clear glass and the densest portions. It is this fact
that we utilize in bromide printing; and it is because we have such
unlimited control over the strength of our light that it is possible with
it to get equally good prints from a wide range of negatives. It is very
much simpler and more practicable to regulate the strength of the light by
increasing or diminishing its distance than by interposing sheets of
paper, ground glass, or opal, as is occasionally done with other
processes.

The necessity, however, for occasionally changing the strength of our
light in this manner may seem to introduce an element of uncertainty into
the problem of exposure; but there is another rule which brings it back
again to simplicity itself, and enables us to quickly calculate equivalent
exposures at varying distances from the light-source. This rule is: "The
intensity of illumination varies inversely as the square of the distance
from the source of light." For instance if a given negative requires five
seconds exposure at one foot from the light, it will have an equivalent
exposure if exposed for twenty seconds at two feet, the square of one
being one, and of two being four.

It remains then only to apply these two rules to our actual work with
bromide paper. The shadows in a certain negative will receive full
exposure, say, in eight seconds at one foot from the light; but the high
lights of the negative are so dense that no light will penetrate them at
that distance from the light in that length of time. Hence a stronger
light must be used, or the action of the same light continued for a longer
time; but the latter will not do since the effect would be to over-expose
the shadows. Hence, knowing that a strong light overcomes contrasts, we
move the negative to the distance of six inches, where the rule tells us
the equivalent exposure will be one-fourth that at twelve inches, in this
case two seconds. Here the shadows get no more light, but it is possible
that the high lights of the negative will be penetrated by reason of the
additional force of the light.

On the other hand we have a thin, flat negative requiring for the shadows
two seconds exposure at one foot from the light. Knowing that a weak light
increases contrasts we move the negative three feet from the light, and
instead of two, give eighteen seconds exposure, the rule telling us that
this is equivalent. Thus we a