Dyes and Pigmknt.s for .Maxwell Disks.     27

for overwashes of the light blue merely sink through
and darken the color without improving the hue. A
moderately saturated solution of the light blue should
be applied first, and when this is dry covered with one
or more rather thin washes of the permanent blue or
new blue).

Hues between blue and violet.—Winsor and Newton's
permanent blue and some of the more violet-hued arti¬
ficial ultramarines, the hues nearer violet washed with
crystal violet or gentian violet.

Violet.—Crystal violet.

Hues between violet and red.—Methyl violet lb. washed

with rhodamin b.; for hues nearer red, rhodamin b. with
'Diivoe''s geranium red (dry) or croceiti scarlet b.

While more or less similar in hue to rhodamin b.,
several other aniline dyes, as acid fuchsin, rubin s.,
rosein, magenta, etc., do not combine satisfactorily with
the violets, the mixture soon becoming dark or dull and
none of them are quite as pure a purple or red-violet.

It is most important to remember that disks thus
colored must be carefully protected from light when not
in actual use and never exposed to direct sunlight. The
artificial ultramarines are, of course, permanent, and so,
practically, are crocein scarlet, gold orange, orange g,,
and auramin—that is to say, are not materially affected
by the action of light except after very prolonged expo¬
sure, though the last named undergoes a change of hue:
but the green and violet aniline dyes are all very evanes¬
cent, rapidly fading and eventually disappearing; light
blue and rhodamin, while sensitive to light, are far less
so than the greens and violets.