and so giving rise to
other tints, farther in. In this way there comes into existence that
iris-like border which is now and then seen.

The occasional upsetting of the crystals, which is required to keep
them fluttering, may be produced in any of three ways. The cloudlets
may have been formed from the blending together of two layers of air
saturated at different temperatures, and moving with different
velocities or in different directions. Where these currents intermix,
a certain amount of disturbance will prevail, which, if sufficiently
slight, would not much interfere with the regularity of the crystals,
and might yet be sufficient to occasion little draughts, which would
blow them about when formed. Or, if the cold layer is above, and if it
is in a sufficient degree colder, there need not be any previous
relative motion of the two layers; the inevitable convection currents
will suffice. Another, and probably the most frequent, cause for
little breezes in the neighborhood of the cloudlets is that when the
cloudlets are formed they immediately absorb the heat of the sun in a
way that the previously clear air had not done. If they absorb enough,
they will rise like feeble balloons, and slight return currents will
travel downward round their margins, throwing all crystals in that
situation into disorder.

I do not include among the causes which may agitate the crystals
another cause which must produce excessively slight currents of air,
namely, that arising from the subsidence of the cloudlets owing to
their weight. The crystals will fall faster wherein cloud masses than
in the intervening portions where the cloud is thinner. But the
subsidence itself is so slow that any relative motions to which
differences in the rate of subsidence can give rise are probably too
feeble to produce an appreciable effect. Of course, in general, more
than one of the above causes will concur; and it is the resultant of
the effects which they would have separately produced that will be
felt by the crystals.

If the precipitation had taken place so very evenly over the sky that
there were no cloudlets formed, but only one uniform veil of haze,
then the currents which would flutter the crystals may be so entirely
absent that the little plates of crystals can fixedly assume the
horizontal position which is natural to them. In this event the cloud
will exhibit no iridescence, but, instead of it, a vertical circle
through the sun will prese