on the eye entirely in judging
the quality of the light, it will sometimes play you tricks. The rays
which are most active on the plates are those which have the least effect
on the eye. We can, however, by chemical means arrive at an exact
estimate of the active power, and for this purpose an actinometer is
used. This is simply an arrangement whereby a piece of sensitized paper
is exposed and allowed to darken to a standard tint, and by the time it
takes to reach that tint the value of the light is judged. Capt. Abney
has, however, pointed out that ordinary sensitized paper is not suitable
for bromide plates, since there are conditions of light in which the
plates will be fairly rapid while the paper will be very slow. He gives a
formula for a bromide paper, which is treated with tannin in order to
absorb the bromine set free during exposure, otherwise the darkening
would be very slight. I used this paper for a while, but found it rather
slow. The tannin also turned brown on keeping for a week or so. I then
made some more, substituting for tannin potassium _nitrite_ (not
nitrate), which is colorless. This was an improvement, but still it was
just slow enough.

However, noticing in Capt. Abney's article the statement that the bromide
of silver should be as nearly as possible in the same state in the paper
as in the plate, I thought "Why not Morgan's paper?" This, of course, is
just bromide emulsion on paper, and if, as I suspect from its color, it
contains a trace of iodide, why, so do most commercial plates. A sheet of
this paper cut into strips, soaked for ten minutes in a fifteen-grain
solution of potassium nitrite, and dried, gives a sensitive paper which
darkens with great rapidity to a good deep tint, and keeps indefinitely.
Here is some prepared last summer, which is still quite good. To use this
paper make a little box so that a little roll of it can be stored in one
end, and drawn forward as required beneath a piece of glass.

Bearing in mind that your table of exposures is calculated for the best
spring light, go to the country some bright day next month with
note-book, actinometer, and the necessary appliances for exposing a few
plates. Select, say, an open landscape, and use your smallest stop. When
all ready to expose, get out your actinometer and expose it to the
reflected light of the sky for ten seconds (if the sun is shining, turn
your back to it, and keep the actinometer in your own shadow); then put