Soon in the west the light faded; but there came out of the east a lovely flush, and the general sky was presently flamboyant with afterglow. The front set of clouds was darker except on the edges, the red being on the clouds behind; and the horizon in the east was particularly rich with dark red hues.
Gradually the eastern glow rose and reddened all the clouds, but the front clouds were still grey. The effect was very fine in contrast. The fleecy clouds overhead became transparently light red, as they stretched over to reach the silver-streaked west. The new moon was just appearing upright against a slightly less bright opening in the sky, betokening the firm hardness of autumn.
Soon the colouring melted away, and the peaceful reign of the later twilight possessed the land.
Now why that brilliancy of the east, when the west was colourless? Most of all you note the immense variety and wealth of reds. These are due to dust in the atmosphere. We are the more convinced of this by the very remarkable and beautiful sunsets which occurred after the tremendous eruption at Krakatoa, in the Straits of Sunda, thirty years ago. There was then ejected an enormous quantity of fine dust, which spread over the whole world’s atmosphere. So long as that vast amount of dust remained in the air did the sunsets and afterglows display an exceptional wealth of colouring. All observers were struck with the vividly brilliant red colours in all shades and tints.
The minute particles of dust in the atmosphere arrest the sun’s rays and scatter them in all directions; they are so small, however, that they cannot reflect and scatter all; their power is limited to the scattering of the rays at the blue end of the spectrum, while the red rays pass on unarrested. The display of the colours of the blue end are found in numberless shades, from the full deep blue in the zenith to the greenish-blue near the horizon.
If there were no fine dust-particles in the upper strata, the sunset effect would be whiter; if there were no large dust-particles, there would be no colouring at all. If there were no dust-particles in the air at all, the light would simply pass through into space without revealing itself, and the moment the sun disappeared there would be total darkness. The very existence of our twilight depends on the dust in the air; and its length depends on the amount and extension upwards of the dust-particles.
But how have the particles been increased in size in the east? Because, as the sun was sinking, but before its rays failed to illumine the heavens, the temperature of the air began to fall. This cooling made the dust-particles seize the water-vapour to form haze-particles of a larger size. The particles in the east first lose the sun’s heat, and first become cool; and the rays of light are then best sifted, producing a more distinct and darker red. As the sun dipped lower, the particles overhead became a turn larger, and thereby better reflected the red rays. Accordingly, the roseate bands in the east spread over to the zenith, and passed over to the west, producing in a few minutes a universal transformation glow.
To produce the full effect often witnessed, there must be, besides the ordinary dust-particles, small crystals floating in the air, which increase the reflection from their surfaces and enhance the glow effects. In autumn, after sunset, the water-covered dust-particles become frozen and the red light streams with rare brilliancy, causing all reddish and coloured objects to glow with a rare brightness. Then the air glows with a strange light as of the northern dawn. From all this it is clear that, though the colouring of sunset is produced by the direct rays of the sun, the afterglow is produced by reflection, or, rather, radiation from the illuminated particles near the horizon.
The effect in autumn is a stream of red light, of varied tones, and rare brilliancy in all quarters, unseen during the warmer summer. We have to witness the sunsets at Ballachulish to be assured that Waller Paton really imitated nature in the characteristic bronze tints of his richly painted landscapes.