The Colour of the Sky

Alfred Russel Wallace

If we look at the sky on a perfectly fine summer’s day we shall find that the blue colour is the most pure and intense overhead, and when looking high up in a direction opposite to the sun. Near the horizon it is always less bright, while in the region immediately around the sun it is more or less yellow. The reason of this is that near the horizon we look through a very great thickness of the lower atmosphere, which is full of the larger dust particles reflecting white light, and this dilutes the pure blue of the higher atmosphere seen beyond. And in the vicinity of the sun a good deal of the blue light is reflected back into space by the finer dust, thus giving a yellowish tinge to that which reaches us reflected chiefly from the coarse dust of the lower atmosphere. At sunset and sunrise, however, this last effect is greatly intensified, owing to the great thickness of the strata of air through which the light reaches us. The enormous amount of this dust is well shown by the fact that then only we can look full at the sun, even when the whole sky is free from clouds and there is no apparent mist. But the sun’s rays then reach us after having passed, first, though an enormous thickness of the higher strata of the air, the minute dust of which reflects most of the blue rays away from us, leaving the complementary yellow light to pass on. Then, the somewhat coarser dust reflects the green rays, leaving a more orange-coloured light to pass on; and finally some of the yellow is reflected, leaving almost pure red. But owing to the constant presence of air currents, arranging both the dust and vapour in strata of varying extent and density, and of high or low clouds which both absorb and reflect the light in varying degrees, we see produced all those wondrous combinations of tints and those gorgeous ever-changing colours which are a constant source of admiration and delight to all who have the advantage of an uninterrupted view to the west and who are accustomed to watch for those not infrequent exhibitions of nature’s kaleidoscopic colour painting. With every change in the altitude of the sun the display changes its character; and most of all when it has sunk below the horizon, and owing to the more favourable angles a larger quantity of the coloured light is reflected toward us. Especially when there is a certain amount of cloud is this the case. These, so long as the sun was above the horizon, intercepted much of the light and colour; but when the great luminary has passed away from our direct vision, his light shines more directly on the under sides of all the clouds and air strata of different densities; a new and more brilliant light flushes the western sky, and a display of gorgeous ever-changing tints occurs which are at once the delight of the beholder and the despair of the artist. And all this unsurpassable glory we owe to—dust!

译文：

天空的色彩
晴空万里的夏日，如果我们观察一下天空，且背向太阳，极目仰望，就会发现头顶上空的蓝色最纯净，最浓郁。靠近天边，色彩往往较暗淡，太阳周围的地方则略呈 黄色。这是因为我们向天边望去时，目光要穿过极厚的低空大气层，其中布满颗粒较大的尘埃，反射出白光，这就冲淡了天际高空大气层的纯蓝色。在太阳附近，大 量蓝光则由细微的尘埃反射回太空。这样，主要由低空大气层的粗粒尘埃反射到地面的光线，便带有浅黄色。不过，在日出日落时，由于光线到达地面南非要穿过厚 厚的大气层，这种反射效果大大增强了。只有在这种时候，我们才可以直视太阳，即使万里长空没有一点云彩，不见一丝雾霭。这就充分显示了

低空尘埃的数量之大。但是太阳的光线终于到达了地面。它们先是穿过厚度极大的高空大气层，其中的细微尘埃把大部分蓝色的光反射掉了，让补色的黄光继续通行。然后，粗粒尘埃又反 射掉绿色的光，让偏橙色的光继续通行。最后，部分黄色的光也反射掉，剩下几乎是纯红色的了。不过，由于不断出现气流，把法埃与水汽分层排列，广度不均，密 度各异，加上高低空常有云层，不同程度地吸收并反射阳光，我们这才看到各种奇异的色调斑剥陆离，诸多绚丽的色彩变化万千；任何人只要有幸将西方的景致一览 无余，只要有心观看大自然不时展现的那一幅幅瞬息万变的彩画，都会为之赞不绝口，喜不自胜。随着夕阳缓缓西坠，这种景观也不断变幻；尤其是在太阳沉入地平 线之后，由于角度更加适宜，五颜六色的光就都发射到地面上来。遇有些许云雾，更是如此。本来，只要太阳还位于地平线之上，云雾便截住了不少夕阳和色彩；而 今太阳从我们的视野消失，阳关便更为直接地照射到密度各异的重重云霭与层层大气的底部；一片崭新的、更加灿烂的阳光染红了西天，一幅景观色彩绚丽，变化万 千，观赏者固然赏心悦目，然而自叹莫及。而我们之所以能领略如此无与伦比的美景，全应归功于—尘埃！