Marvellous sunsets are to be witnessed from the mountains of the New World. The following is a short and graphic description of sunset glories on the Sierra Nevada Mountains by Mr. Clarence King, whose name is well known to geologists:—
"While I looked, the sun descended, shadows climbed the Sierras, casting a gloom over foothill and pine, until at last only the snow summits, reflecting the evening light, glowed like red lamps along the mountain-wall for hundreds of miles. The rest of the Sierra became invisible. The snow burned for a moment in the violet sky, and at last went out."
These marvellous effects appeal powerfully to our sense of beauty and produce in most minds feelings of intense delight; but they also appeal to the reasoning faculty in man, and an intelligent observer naturally inquires, "Why are these things so? How are those glorious colours of crimson, orange, and yellow produced?" A full explanation cannot be attempted here; but this much may perhaps be said without tiring the patience of the reader. White light, such as sunlight or the light from an electric arc, is composed of all the colours of the rainbow,—violet, indigo, blue, green, yellow, orange, and red. A ray of sunlight on passing through a prism is split up into all these colours in the above order, and we get them arranged in a band which is known as the spectrum. Thus it is proved that white light is made up of all colours (black is not a colour, but the absence of colour). Now, when the sun is low down in the sky, as at sunset, only some of these colour-rays are able to pass through the atmosphere and so to reach our eyes, while others are stopped in passing through very many miles of atmosphere (as they must obviously do when the sun is low). Those which are stopped are the blue rays and others allied to blue, such as purple and green; but the red and yellow rays are able to pass on till they come to us. Hence red, yellow, and orange are the prevailing sunset tints.
What, then, becomes of the missing blue rays? They are caught by the myriads of little floating particles in the air, and reflected away from us. That is why we do not see them; their course is turned back, just as waves breaking against a stone sea-wall are turned back or reflected. A person situated behind such a wall will not see the waves which break against it; but suppose a very big wave came: it would come right over, and then we should soon become aware of its presence. So it is with the little waves of light: some are stopped and turned back as they break against the myriads of little dust particles and the still more numerous particles of mist always floating in the air; while others, which are larger, break over them and travel on undisturbed until they reach our eyes. Now, the larger waves of light are the red waves, while the smaller ones are the blue waves; hence there is no difficulty in understanding why the red waves (or vibrations) are seen at sunset and sunrise, to the exclusion of the blue waves. But it must be borne in mind that light-waves are of infinitesimal smallness, thousands and thousands of them going to make up an inch. Sound also travels in waves, and the phenomena of sound serve to illustrate those of light; but sound-waves are very much larger.
The reason why the sky overhead appears blue is that we see the blue rays reflected down to the earth from myriads of tiny dust and water particles, while the red rays pass on over our heads, which is just the reverse of what happens at sunset.
On the southern slopes of the Alps the blues of the sky are generally very different from those on the northern side; and this is probably due to the greater quantity of water-vapour in the air, for the moist winds come from the south. Sunrises in the Alps are quite as glorious to behold as sunsets; but comparatively few people rise early enough to see them. Speaking generally, it may be said that in Alpine sunrises the prevailing colours are orange and gold, in sunsets crimson or violet-pink. After a cool night the atmospheric conditions will obviously be different from those which exist after a warm day, and more water-vapour will have been condensed into mist or cloud. Hence we should expect a somewhat different effect.
The snowfields on high ranges of mountains are of a dazzling whiteness; and their bright glare is so great as to distress the eyes of those who walk over them without blue glasses, and even to cause inflammation. At these heights the traveller is not only exposed to the direct rays of the sun, untempered save for a thin veil of rarefied air, but also to an intense glare produced by the little snow-crystals which scatter around the beams of light falling upon them. Scientific men, who have studied these matters, say that the scorching of the skin and "sun-burning" experienced by Alpine travellers is not caused, as might be supposed, by the heat of the sun, but by the rays of light darting and flashing on all sides from myriads of tiny snow-crystals.
Occasionally a soft lambent glow has been observed on snowfields at night. This is a very curious phenomenon, to which the name of "phosphorescence" has, rightly or wrongly, been given. A pale light may often be seen on the sea during a summer night, when the water is disturbed in any way; and if one is rowing in a boat, the oars seem glowing with a faint and beautiful light. It is well known that this is caused by myriads of little light-producing animalcules in the sea-water. But we can hardly suppose that the glow above referred to is produced by a similar cause. One observer says the glow is "something like that produced by the flame of naphtha;" and he goes on to say that at every step "an illuminated circle or nimbus about two inches in breadth surrounded our feet, and we seemed to be ploughing our way through fields of light, and raising clods of it, if I may be allowed the expression, in our progress." Another observer, also an Alpine traveller, says that at almost every footstep the snowy particles, which his companion in front lifted with his feet from the freshly fallen snow, fell in little luminous showers. The exact cause which produces this strange effect at night has not been ascertained.
There is another curious phenomenon often seen just before sunset on a mountain in Hungary. It is known as "The Spectre of the Brocken." The Brocken is the highest summit of the Hartz Mountains. As you step out upon the plateau upon the top of the hill, your shadow, grim and gigantic, is apparently flung right out against the eastern sky, where it flits from place to place, following your every movement. The explanation is simply this: to the east of the Hartz Mountains there is always a very dense and hazy atmosphere, so dense that it presents a surface capable of receiving the impression of a shadow, and of retaining it, as a wall does. The shadows are really close at hand, not a long way off, as might at first sight be supposed. If very far away, they would be too faint to be visible.