If the floating film be now caught upon a plate which has been
slightly waxed, and it is allowed to dry on this plate, it will become
quite flat and free from wrinkles. To wax the plate, it should be held
before the fire until it is moderately hot, after which it is rubbed
over with a lump of wax, and the excess is polished off with a piece
of flannel. When the film is dry, it will leave the waxed glass
immediately, if one corner is lifted by means of a penknife. The film
will become somewhat enlarged during the above-described operation; but,
by taking suitable precautions, this enlargement may be avoided. It is
also convenient to prepare the hydrofluoric acid extemporaneously by the
action of sulphuric acid on fluoride of sodium; and, in many cases, it
is advisable to thicken up the film by an additional layer of gelatine.
The following directions embody these points. The negative, which must
be unvarnished, is leveled, and covered with a layer of warm gelatine
solution (one in eight) about as thick as a sixpence.
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