During the dyeing process, the coloring matter
combines with the chromium fixed on the fiber, and thus develops the
colored photograph.
The prints in Prussian blue are produced in a similar manner, the
sensitive salt with which the calico is prepared being ammonium
ferricitrate, and the developer potassium ferricyanide.
Investigation has shown that the most chemically active rays are those
situated at the blue end of the solar spectrum; and although all the
rays absorbed by a sensitive colored body affect its change, it is
doubtless the blue rays which are the chief cause of the fading of
colors. Experiments are on record, indeed, which prove this.
Depierre and Clouet (1878-82) exposed a series of colors, printed and
dyed on calico, to light which had passed through glasses stained red,
orange, yellow, green, blue, and violet, corresponding to definite
parts of the spectrum. They found that the blue light possessed the
greatest fading power, red light the least.
More recently (1886-88) Abney and Russell exposed water colors under
red, green, and blue glass, and came to the same conclusion.
But the chemical energy of the sun's rays is not the sole cause of the
fading of colors. There are certain contributory causes as important
as the light itself.
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