CHAPTER XX

THE RAINBOW

The poet Wordsworth rapturously exclaimed--

"My heart leaps up when I behold A rainbow in the sky."

And old and young have always been enchanted with the beautiful phenomenon. How glorious is the parti-coloured girdle which, on an April morning or September evening, is cast o'er mountain, tower, and town, or even mirrored in the ocean's depths! No colours are so vividly bright as when this triumphal arch bespans a dark nimbus: then it unfolds them in due prismatic proportion, "running from the red to where the violet fades into the sky."

A plain description of the formation of the rainbow is not very easily given, but a short sketch may be useful. Beautiful as is the ethereal bow, "born of the shower and colour'd by the sun," yet the marvellous effect is more exquisitely intensified in its gorgeous display when the hand of science points out the path in which the sun's rays, from above the western horizon, fall on the watery cloud, indicating fine weather--"the shepherd's delight."

One law of reflection is that, when a ray of light falls on a plane or spherical surface, it goes off at the same angle to the surface as it fell. One law of refraction is that, when a ray of light passes through one medium and enters a denser medium (as from air to water), it is bent back a little. By refraction you see the sun's rays long after the sun has set; when the sun is just below the horizon, an observer, on the surface of the earth, will see it raised by an amount which is generally equal to its apparent diameter.

The rays of different colours are bent back (when passing through the water) at different rates, some slightly, others more, from the red to the violet end. The rainbow, then, is produced by refraction and reflection of the several coloured rays of sunlight in the drops of water which make up falling rain.

The sun is behind the observer, and its rays fall in a parallel direction upon the drops of rain before him. In each drop the light is dispersively refracted, and then reflected from the farther face of the drop; it travels back through the drop, and comes out with dispersing colours.

According to the height of the sun, or the slope of its rays, a higher or lower rainbow will be formed. And, strange, no two people can see the very same bow; in fact the rainbow, as seen by the one eye, is not formed by the same water-drops as the rainbow seen by the other eye.

When the primary bow is seen in most vivid colours on a dark cloud, a second arch, larger and fainter, is often seen. But the order of the colours is quite reversed. At a greater elevation, the sun's ray enters the lower side of a drop of rain-water, is refracted, reflected _twice_, and then refracted again before being sent out to the observer's eye. That is why the colours are reversed.

_A one-coloured rainbow_ is a curious and rare phenomenon. It is a strange paradox, for the very idea of a rainbow brings up the seven colours--red, orange, yellow, green, blue, indigo, and violet. Yet Dr. Aitken tells us of a rainbow with one colour which he observed on Christmas Day, in 1888.

He was taking his walk on the high ground south of Falkirk. In the east he observed a strange pillar-like cloud, lit up with the light of the setting sun. Then the red pillar extended, curved over, and formed a perfect arch across the north-eastern sky. When fully developed, this rainbow was the most extraordinary one which he had ever seen. There was no colour in it but red. It consisted simply of a red arch, and even the red had a sameness about it.

Outside the rainbow there was part of a secondary bow. The Ochil Hills were north of his point of observation. These hills were covered with snow, and the setting sun was glowing with rosy light. Never had he seen such a depth of colour as was on them on this occasion. It was a deep, furnacy red. The sun's light was shorn of all the rays of short-wave length on its passage through the atmosphere, and only the red rays reached the earth. The reason why the Ochils glowed with so deep a red was owing to their being overhung by a dense curtain of clouds, which screened off the light of the sky. The illumination was thus principally that of the direct softer light of the sun.