by flowing the plate with solutions of nitric acid and ammonium
sulphide. Intensification may also be accomplished by immersing
the plate in a solution of mercuric chloride until the negative is
bleached and then flowing with dilute nitric acid, followed by am
monium sulphide. In either case, if the negative be properly in
tensified, it will have the image represented by lines of clear glass
while the surrounding parts will be black, opaque masses.
Half Tone Negatives.—in reproducing line drawings, merely
blacks and whites are to be dealt with, but when photographs, and
wash drawings are to he reproduced every gradation from pure
white to solid black is met with. For such purposes the ordinary
line process will avail nothing and for a long time photographs
and wash drawings were only reproducible by hand engraving or
by line process reproductions of pen and ink drawings of the orc
iginal. This was tedious, somewhat expensive and often unsatis
factory but the half-tone process enables now the reproduction of
a photograph almost as perfectly as the photograph' itself.
The wood engraver, and also the draughtsman, in representing
surfaces uses lines, varying their weight and the spaces between
them as the parts which he wishes to represent be light or dark
and he will sometimes use stipple—dots—to represent other sur
faces. By the skillful use of these methods he is able to interpret
to the eye the various values of light and shade and even of color.
It was this that suggested the solution of the problem of repro
ducing half tones. •
If the image on the negative could be broken up in such a
manner that it be formed of dots and clear spaces, the size of the
dots varying iu correspondence with the different shades in the
copy, then the negative could be used for printing on. metal.
This was accomplished by placing, in the camera, between the
lens and the •sensitive film, a screen plate—consisting of a glass
plate ruled with fine parallel opaque lines—having its surface
broken up into alternate clear and opaque spaces. Ordinarily
the screen plate used has lines 'ruled in two directions, one set at
right angles to the other, it being formed by sealing together two
single line screens. The screen plate is designated by the num
ber of lines per unit length, varying, in general, from one hun
dred to one hundred and seventy-five lines per inch. The action
of such a screen, when placed in front of the sensitive plate dur
ing exposure is to cause the negative image to be formed of sep-
PHOTO-V,NGRAVING