l, or flowers; but the _force_ of purple and blue in some
butterflies, and the methods of clouding, and strength of burnished
lustre, in plumage like the peacock's, give them more universal
interest; in some birds, also, as in our own kingfisher, the colour
nearly reaches a floral preciousness. The lustre in most, however, is
metallic rather than vitreous; and the vitreous always gives the purest
hue. Entirely common and vulgar compared with these, yet to be noticed
as completing the crystalline or vitreous system, we have the colours of
gems. The green of the emerald is the best of these; but at its best is
as vulgar as house-painting beside the green of bird's plumage or of
clear water. No diamond shows colour so pure as a dewdrop; the ruby is
like the pink of an ill-dyed and half-washed-out print, compared to the
dianthus; and the carbuncle is usually quite dead unless set with a
foil, and even then is not prettier than the seed of a pomegranate. The
opal is, however, an exception. When pure and uncut in its native rock,
it presents the most lovely colours that can be seen in the world,
except those of clouds.

We have thus in nature, chiefly obtained by crystalline conditions, a
series of groups of entirely delicious hues; and it is one of the best
signs that the bodily system is in a healthy state when we can see these
clearly in their most delicate tints, and enjoy them fully and simply,
with the kind of enjoyment that children have in eating sweet things.

174. Now, the course of our main colour schools is briefly this:--First
we have, returning to our hexagonal scheme, line; then _spaces_ filled
with pure colour; and then _masses_ expressed or rounded with pure
colour. And during these two stages the masters of colour delight in the
purest tints, and endeavour as far as possible to rival those of opals
and flowers. In saying "the purest tints," I do not mean the simplest
types of red, blue, and yellow, but the most pure tints obtainable by
their combinations.

175. You remember I told you, when the colourists painted masses or
projecting spaces, they, aiming always at colour, perceived from the
first and held to the last the fact that shadows, though of course
darker than the lights with reference to which they _are_ shadows, are
not therefore necessarily less vigorous colours, but perhaps more
vigorous. Some of the most beautiful blues and purples in nature, for
instance, are those of mountains in shadow