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.

About fifty years ago, Chevreul showed what these accessory causes
are, by exposing to light a number of dyed colors under varied
conditions, e.g., in a vacuum, in dry and moist hydrogen, dry and
moist air, water vapor, and the ordinary atmosphere. He found that
such fugitive colors as orchil, safflower, and indigo-carmine fade
very rapidly in moist air, less rapidly in dry air, and that they
experience little or no change in hydrogen or in a vacuum. The general
conclusion arrived at was, that light, when acting alone, i.e.,
without the aid of air and moisture, exercises a very feeble
influence. Further, it was determined that the air and moisture,
without aid of light, have also comparatively little effect on dyed
colors. Abney and Russell, in their experiments with water colors,
obtained similar results.