n of bromide of potassium--containing one hundred grains
of that salt dissolved in eight or ten ounces of distilled water. The
picture is again washed with distilled water, and then finally dried.
Instead of bromide of potassium, a solution of hyposulphite of soda, as
before directed, may be used with equal advantage.

The original calotype picture, like the photographic one described in
the last chapter, is negative, that is to say, it has its lights and
shades reversed, giving the whole an appearance not conformable to
nature. But it is easy from this picture to obtain another which shall
be conformable to nature; viz., in which the lights shall be
represented by lights, and the shades by shades. It is only necessary
to take a sheet of photographic paper (the bromide paper is the best),
and place it in contact with a calotype picture previously rendered
transparent by wax or oil as before directed. Fix it in the frame,
Fig. 29, expose it in the sunshine for a short time, and an image or
copy will be formed on the photogenic paper. The calotype paper itself
may be used to take the second, or positive, picture, but this Mr.
Talbot does not recommend, for although it takes a much longer time to
take a copy on the photogenic paper, yet the tints of such copy are
generally more harmonious and agreeable. After a calotype picture has
furnished a number of copies it sometimes grows faint, and the
subsequent copies are inferior. This may be prevented by means of a
process which revives the strength of the calotype pictures. In order
to do this, it is only necessary to wash them by candlelight with
gallo-nitrate of silver, and then warm them. This causes all the
shades of the picture to darken considerably, while the white parts are
unaffected. After this the picture is of course to be fixed a second
time. It will then yield a second series of copies, and, in this way,
a great number may frequently be made.

The calotype pictures when prepared as we have stated, possess a
yellowish tint, which impedes the process of taking copies from them.
In order to remedy this defect, Mr. Talbot has devised the following
method. The calotype picture is plunged into a solution consisting of
hyposulphite of soda dissolved in about ten times its weight of water,
and heated nearly to the boiling point. The picture should remain in
about ten minutes; it must then be removed, washed and dried. By this
process the picture is rendere