Camera Work: A Photographic Quarterly (1908 (Heft 22))

of the blue and yellow. A negative made in this manner represents red as
black, vermilion a little grayer, orange and violet still lighter, while yellow,
blue and green are the lightest tones. This plate contains the red element
of the picture. The red glass absorbs the blue and blue-green, rendering it
very dark in the photograph, while pure yellow will appear white, yellowish-
green and orange being represented as light grays—this, then, is the blue
picture. For the picture made with the blue filter an ordinary photographic
plate can be used, but for the green-filter negative an orthochromatic plate,
sensitive to green and yellow, is necessary. Bathing a photographic plate
in certain dyes, or mixing the dyes with the emulsion, has the property of
making them more sensitive to color. A plate bathed in a solution of
erythrosine becomes very sensitive to green and yellow, and such a plate
could be used with the green filter to produce the red printing picture. A
plate bathed in pinachrome or ethyl red is very sensitive to red and orange,
and used with the red filter it gives us the blue printing-plate.
If the three pictures of the color chart above described be printed on thin
transparent film, each in one of the three fundamental pigment-colors ; red,
blue, and yellow—colors complementary to the primary colors which acted
as fiiters; and then these pictures be superimposed, the result will be a
reproduction of the original color chart.
This is roughly and theoretically the basis of all three-color photog-
raphy, which produces an image in color by the mixing of pigments, and it
is termed the subtractive synthesis, for by each mixture color is subtracted
or absorbed, and as in painting, each mixture is a step towards black, If we
take the three original screens—the red-orange, the green, and the blue-violet
glasses—and project their colors by three lanterns on a screen, so that they
partially overlap each other, we find that where the green and orange overlap
they produce yellow ; where green and violet overlap, the result is blue;
and where violet and orange overlap, the result is pure red. Where the
three colors overlap, white is the result. This is called the additive synthesis,
and each mixture of color is a step towards white light.
By projecting transparent positives of the three negatives made from the
color chart, with three lanterns (fitted with the original color filters), so that
the three images overlap each other perfectly, we again obtain a reproduction
of the color chart, this time by the additive synthesis. In the blue screen
positive, yellow is represented as dark, consequently no light from the lantern
with the yellow filter reaches the screen at that point; whereas in other
positives yellow is represented by practically clear glass; so that both the
orange-red and the green rays impinge at this point on the screen,
and produce yellow. In a like manner the other colors are produced.
A very ingenious instrument, the chromoscope, for producing and
showing color photographs by this method, was invented in 1892 by
Frederick E. Ives.
In the Autochrome plate the picture is produced by this synthesis, but
instead of making three exposures and three negatives through different
screens, the plate itself contains the screen, in the form of microscopic

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