each gradation of light and shade affects the sensitive film corre
spondingly. Therefore, if a properly exposed half tone plate be
developed it will be found, on examination, that those parts cor
responding to the extra high lights will be almost black, the next
highest lights will be dark with minute transparent spots regu
larly spaced in the mass, and, passing through the half tones, it
will be observed that the plate appears ruled with transparent
parallel lines, corresponding to the opaque lines of the screen
plate, and leaving isolated regularly spaced black spots. Then,
passing to those parts corresponding to the deep shadows it will
be observed that the dark spots diminish in size until they are en
tirely absent in the deepest shadows of the picture.
If a picture be printed from such a negative we would obtain a
result similar to that of a half tone illustration. Further than the
use of the screen plate and its effect upon the negative the pro
cess of making a half tone negative is similar to that of making a
line negative.
Reversal of the Negative.•—There is, however, one more opera
tion, which both line and half tone negatives must undergo
after intensification if they are to be used in making illustrations,
and that is, reversal of the negative. If the negative be used as
it is formed by the camera the resulting illustration, printed by
the press, will have right and left reversed, and to prevent this
the film is stripped from the glass plate and placed upon another
plate in reverse.
Printing the Negative Image upon Metal.—The principle en
abling the production of metal engravings by photography is this:
Certain organic substances mixed with a bichromate will, when
dry, be rendered insoluble by the action of actinic light rays.
Hence, if a metal plate be coated with such a film, be allowed to
dry, and then be exposed to light under a negative, the light pass
ing through the clear spaces will render insoluble the chromated
film directly beneath, while those parts unaffected—parts pro
tected by the opaque masses of the negative—will remain soluble.
Washing the printed metal plate removes the soluble parts leav
ing the image, on the metal, formed of the insoluble coating.
This explains why such particular care was exercised throughout
the process of negative making to obtain a negative image com
posed of transparent and opaque parts; the chromated compound
is either sol Able or insoluble; therefore, to reproduce the picture