d then disturbed from the horizontal position. If this
occasionally happens, the crystals will keep fluttering, and at any
one moment some of them will be turned so as to reflect a ray from the
sun to the eye of the observer from the flat surface of the crystal
which is next him. Now, if the conditions are such as to produce
crystals which are plates with parallel faces, and as they are also
transparent, part only of the sun's ray that reaches the front face of
the crystal will be reflected from it; the rest will enter the
crystal, and, falling on the parallel surface behind, a portion will
be there reflected, and passing out through the front face, will also
reach the eye of the observer.

These two portions of the ray--that reflected from the front face and
that reflected from the back--are precisely in the condition in which
they can interfere with one another, so as to produce the splendid
colors with which we are familiar in soap bubbles. If the crystals are
of diverse thicknesses, the colors from the individual crystals will
be different, and the mixture of them all will produce merely white
light; but if all are nearly of the same thickness, they will transmit
the same color toward the observer, who will accordingly see this
color in the part of the cloud occupied by these crystals. The color
will, of course, not be undiluted; for other crystals will send
forward white light, and this, blended with the colored light, will
produce delicate shades in cases where the corresponding colors of a
soap bubble would be vivid.

We have now only to explain how it happens that on very rare occasions
the colors, instead of lying in irregular patches, form definite
fringes round the borders of the cloudlets. The circumstances that
give rise to this special form of the phenomenon appear to be the
following: While the cloud is in the process of growth (that is, so
long as the precipitation of vapor into the crystalline state
continues to take place), so long will the crystals keep augmenting.
If, then, a cloudlet is in the process of formation, not only by the
springing up of fresh crystals around, but also by the continued
growth of the crystals within it, then will that patch of cloud
consist of crystals which are largest in its central part, and
gradually smaller as their situation approaches the outside. Here,
then, are conditions which will produce one color round the margin of
the cloud, and that color mixed with others,