In the
ordinary photograph, distant hills are lost through overexposure, yet the
foreground seems underexposed, and yellow straw-stacks and bright autumn
leaves appear black. In the chlorophyl photograph, the distant hills are
not overexposed, nor is the foreground underexposed; the yellow
straw-stacks appear nearly white, and bright autumn leaves contrast
strongly with the dark green about them.
To test the relative color-sensitiveness of plain emulsion plates, plates
stained with eosine, and plates stained with the blue-myrtle chlorophyl,
I exposed one of each kind through the same yellow screen, giving each
five minutes exposure, on the same piece of copy, which was the
chromo-lithograph already described. The plain emulsion plate showed only
the high lights of the picture, after prolonged development. The eosine
plate was underexposed, but brought up everything fairly well except the
scarlet hat, which came up like black. The chlorophyl plate was
overexposed, brought out all colors better than the eosine plate, and
gave full value to the bright scarlet of the hat, the detail in which was
beautifully rendered.
Dr. Vogel advanced the theory that silver-bromide is insensitive to
yellow and red, because it reflects or transmits those colors; and that
it becomes sensitive when stained, because of the optical properties of
the dyes.
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