19. Prismatic Cloud

But now note, there is another kind of cloud, pure white, and exquisitely delicate; which acts not by reflecting, nor by refracting, but, as it is now called,2 diffracting, the sun’s rays. The particles of this cloud are said—with what truth I know not3—to send the sunbeams round them instead of through them; somehow or other, at any rate, they resolve them into their prismatic element; and then you have literally a kaleidoscope in the sky, with every colour of the prism in 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 splendour.

“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 colour 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 colour 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 colours I ever saw were on those that flew fastest.