Noelting and Herzberg have also observed that the fastness to light,
even of basic colors, e.g., magenta, methyl violet, malachite green,
etc., is increased by a subsequent treatment of the dyed fabric with
copper sulphate solution, although in many cases the color is much
soiled thereby.

Still more recently, A. Scheurer records that by impregnating or
padding certain dyed fabrics with an ammoniacal solution of copper
sulphate, the colors gain considerably in fastness to light. As the
result of his experiments Scheurer concludes that this protective
influence of copper on dyed colors is a general fact, apparently
applicable to all colors; that it is not necessarily due to its action
as a lake-forming substance, since intimate union between the coloring
matter and the copper salt is not necessary. He seems rather inclined
to ascribe its efficacy to the light being deprived of its active rays
during its passage through the oxide of copper.

Knowing, however, the strong reducing action of light in many cases,
and with the absence of positive knowledge concerning the cause of the
fading of colors, it seems to me that the beneficial influence of the
copper may just as probably be due to its well known oxidizing power,
which counteracts the reducing action of the light.

It is interesting to note, in connection with Scheurer's view, that,
many years ago, Gladstone and Wilson (1860) proposed to impregnate
colored materials with some colorless fluorescent substance, e.g.,
sulphate of quinine, evidently with the idea of filtering off the
active ultra-violet rays. How far some such method as this might prove
successful I cannot say, but since we cannot keep our dyed textile
materials in a vacuum, as Chevreul did, nor is it desirable to