elieve, _ice_-clouds;
threads, and meshes, and tresses, and tapestries, flying, failing,
melting, reappearing; spinning and unspinning themselves, coiling and
uncoiling, winding and unwinding, faster than eye or thought can
follow: and through all their dazzling maze of frosty filaments shines
a painted window in palpitation; its pulses of color interwoven in
motion, intermittent in fire,--emerald and ruby and pale purple and
violet melting into a blue that is not of the sky, but of the
sunbeam;--purer than the crystal, softer than the rainbow, and
brighter than the snow.

But you must please here observe that while my first diagram did
with some adequateness represent to you the color facts there
spoken of, the present diagram can only _explain_, not reproduce
them. The bright reflected colors of clouds _can_ be represented in
painting, because they are relieved against darker colors, or, in
many cases, _are_ dark colors, the vermilion and ruby clouds being
often much darker than the green or blue sky beyond them. But in
the case of the phenomena now under your attention, the colors are
all _brighter than pure white_,--the entire body of the cloud in
which they show themselves being white by transmitted light, so
that I can only show you what the colors are, and where they
are,--but leaving them dark on the white ground. Only artificial,
and very high illumination would give the real effect of
them,--painting cannot.

Enough, however, is here done to fix in your minds the distinction
between those two species of cloud,--one, either stationary,[16] or
slow in motion, _reflecting unresolved_ light; the other,
fast-flying, and _transmitting resolved_ light. What difference is
there in the nature of the atoms, between those two kinds of
clouds? I leave the question with you for to-day, merely hinting to
you my suspicion that the prismatic cloud is of finely-comminuted
water, or ice,[17] instead of aqueous vapor; but the only clue I
have to this idea is in the purity of the rainbow formed in frost
mist, lying close to water surfaces. Such mist, however, only
becomes prismatic as common rain does, when the sun is behind the
spectator, while prismatic clouds are, on the contrary, always
between the spectator and the sun.

The main reason, however, why I can tell you nothing yet about
these colors of diffraction or interference, is that, whenever I
try to find anything firm for you to depend on, I am stopped by the
quite