absolute purity; but
above all in force, now, the ruby red and the _green_,--with
purple, and violet-blue, in a virtual equality, more definite than
that of the rainbow. The red in the rainbow is mostly brick red,
the violet, though beautiful, often lost at the edge; but in the
prismatic cloud the violet, the green, and the ruby are all more
lovely than in any precious stones, and they are varied as in a
bird's breast, changing their places, depths, and extent at every
instant.

The main cause of this change being, that the prismatic cloud
itself is always in rapid, and generally in fluctuating motion. "A
light veil of clouds had drawn itself," says Professor Tyndall, in
describing his solitary ascent of Monte Rosa, "between me and the
sun, and this was flooded with the most brilliant dyes. Orange,
red, green, blue--all the hues produced by diffraction--were
exhibited in the utmost splendor.

"Three times during my ascent (the short ascent of the last peak)
similar veils drew themselves across the sun, and at each passage
the splendid phenomena were renewed. There seemed a tendency to
form circular zones of color round the sun; but the clouds were not
sufficiently uniform to permit of this, and they were consequently
broken into spaces, each steeped with the color due to the
condition of the cloud at the place."

Three times, you observe, the veil passed, and three times another
came, or the first faded and another formed; and so it is always,
as far as I have registered prismatic cloud: and the most beautiful
colors I ever saw were on those that flew fastest.

This second diagram is enlarged admirably by Mr. Arthur Severn from
my sketch of the sky in the afternoon of the 6th of August, 1880,
at Brantwood, two hours before sunset. You are looking west by
north, straight towards the sun, and nearly straight towards the
wind. From the west the wind blows fiercely towards you out of the
blue sky. Under the blue space is a flattened dome of earth-cloud
clinging to, and altogether masking the form of, the mountain,
known as the Old Man of Coniston.

The top of that dome of cloud is two thousand eight hundred feet
above the sea, the mountain two thousand six hundred, the cloud
lying two hundred feet deep on it. Behind it, westward and seaward,
all's clear; but when the wind out of that blue clearness comes
over the ridge of the earth-cloud, at that moment and that line, its
own moisture congeals into these white--I b