dissolves the superfluous, unstable compounds, and rapidly clears up the
lighter portions of the picture. On being removed from this, it is
thoroughly washed, dried, and mounted, by pasting it with starch or
dextrine to a card of the proper size.

The reader who has followed the details of the process may like to know
what are the common difficulties the beginner meets with.

The first is in coating the glass with collodion. It takes some practice
to learn to do this neatly and uniformly.

The second is in timing the immersion in the nitrate-of-silver bath. This
is easily overcome; the glass may be examined by the feeble lamp-light at
the end of two or three minutes, and if the surface looks streaky,
replunged in the bath for a minute or two more, or until the surface looks
smooth.

The third is in getting an exact focus in the camera, which wants good
eyes, or strong glasses for poor ones.

The fourth is in timing the exposure. This is the most delicate of all the
processes. Experience alone can teach the time required with different
objects in different lights. Here are four card-portraits from a negative
taken from one of Barry's crayon-pictures, illustrating an experiment
which will prove very useful to the beginner. The negative of No. 1 was
exposed only two seconds. The young lady's face is very dusky on a very
dusky ground. The lights have hardly come out at all. No. 2 was exposed
five seconds. Undertimed, but much cleared up. No. 3 was exposed fifteen
seconds, about the proper time. It is the best of the series, but the
negative ought to have been intensified. It looks as if Miss E.V. had