re enabled to regulate the strength of our
light to suit the character of our negative. But a standard distance of
one foot will not suit with all kinds of lights or with all sizes of
negatives. If, for instance, our light is a Welsbach burner, giving an
intense and comparatively white light, we will find that a normal negative
will print too flat if exposed at one foot. In such a case two or even
three feet would be a better standard. Experience with our light will,
however, furnish the best standard, always taking a standard negative for
the tests.

[Illustration: Fig. 1]

The size of the negative also has its influence on the unit of exposure.
For instance, we may have a half-inch oil-burner, in which case we would
probably have to expose a standard negative at four inches in order to get
the proper contrasts. But this is out of the question with a negative of 5
x 7 or over, as a reference to the diagram, Fig. 1, will clearly show.
Here we find that while the centre of a negative is four inches from the
light the extreme edges will be over five inches from it, the rule as to
intensities telling us that the light at the edges will be only 16/25 of
that at the centre. This would result in a marked falling off of light at
the corners, and would necessitate a constant motion of the printing frame
throughout exposure, which is not wholly satisfactory. The remedy would be
to use a stronger light at a greater distance. But another reference to
Fig. 1 will show that if a 5 x 7 negative be held at seven inches from the
light the difference will be only as 49 is to 56, which can in practice be
disregarded, though it would be better to have it even less. Hence we see
that it is never safe to have our unit less than the base-line of our
plate, and it is better to have it even greater, as we will frequently be
obliged to halve the distance to overcome contrasts. It follows from this
that the larger our negatives the stronger must be our light.

Now all of these considerations may make very dry reading, but the reader
who has followed them closely will see how vital they are to successful
work. It should not be thought, however, that every exposure on bromide
paper must involve an arithmetical calculation. On the contrary, once the
proper distance from the light for the normal negative has been
ascertained, it will be found that nine negatives out of ten will require
no change in the distance from the source of light. This, of co