not, you may
calculate it for yourself by the rules given in the various text books,
provided you have a camera of pretty long focus. However, it will be near
enough for our purpose if you get a sharp image of the sun on a piece of
paper, and while you hold lens and paper, get some one to measure the
distance from the paper to the diaphragm aperture, or, in the case of a
single lens, to the center of the lens. Note down this focal length, and
proceed to measure your diaphragms in sixteenths of an inch.

Then, with pen and paper, proceed to divide the diameter of each stop
into the focus, and state the result as a fraction of the focus, thus
f/8. For example, a Ross half plate rapid symmetrical has a focal length
of 71/2 in.; for convenience reduce this to sixteenths=120. A diaphragm
measuring seven sixteenths will give the fraction f/17. Now let us see if
any of these stops correspond with Mr. Burton's. The first two in his
table will only be found in portrait lenses, but we shall probably find
one to correspond with the third, if we are using a doublet lens; with a
single lens we won't find any so large. Having picked out those that
correspond, and filled in the exposure for them, we have now to deal with
the odd sizes. Here is one, f/27, which is just half way between No. 16
and No. 32, but a moment's thought will show that as the exposure
increases as the square of the diameter, it won't do to take the exposure
half way between the two.

We have another factor to consider now: that is, the rapidity of the
plate. If you use plates by a maker who has a name to sustain, you may be
pretty confident that they are of fairly uniform rapidity, so after you
have got into the way of working any particular brand, the best thing you
can do is to stick to it. The exposures in our table are for plates of
medium rapidity in good spring light. In my own experience I find that
they just suit "thirty times" plates, or fifteen on the sensitometer; but
then I like a full exposure with slow development, and I know that others
find these exposures just right for "twenty times" plates developed in
the usual way. The most rapid plates in the market will not be overdone
with half the given exposures. It must always be borne in mind that an
error of a fraction of a second in either direction may be corrected in
development, and it is impossible to make a very serious error if you
refer to the table.

We come now to the light. If you depend