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 diluter 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，through an enormous thickness of the higher strata of the air，the minute dust of which reflects most 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!
天空的色彩
晴空萬(wàn)里的夏日，如果我們觀察一下天空，且背向太陽(yáng)，極目仰望，就會(huì)發(fā)現(xiàn)頭頂上空的藍(lán)色最純凈，最濃郁。靠近天邊，色彩往往較暗淡，太陽(yáng)周?chē)牡胤絼t略呈黃色。這是因?yàn)槲覀兿蛱爝呁r(shí)，目光要穿過(guò)極厚的低空大氣層，其中布滿顆粒較大的塵埃，反射出白光，這就沖淡了天際高空大氣層的純藍(lán)色。在太陽(yáng)附近，大量藍(lán)光則由細(xì)微的塵埃反射回太空。這樣，主要由低空大氣層的粗粒塵埃反射到地面的光線，便帶有淺黃色。不過(guò)，在日出日落時(shí)，由于光線到達(dá)地面南非要穿過(guò)厚厚的大氣層，這種反射效果大大增強(qiáng)了。只有在這種時(shí)候，我們才可以直視太陽(yáng)，即使萬(wàn)里長(zhǎng)空沒(méi)有一點(diǎn)云彩，不見(jiàn)一絲霧靄。這就充分顯示了低空塵埃的數(shù)量之大。但是太陽(yáng)的光線終于到達(dá)了地面。它們先是穿過(guò)厚度極大的高空大氣層，其中的細(xì)微塵埃把大部分藍(lán)色的光反射掉了，讓補(bǔ)色的黃光繼續(xù)通行。然后，粗粒塵埃又反射掉綠色的光，讓偏橙色的光繼續(xù)通行。最后，部分黃色的光也反射掉，剩下幾乎是純紅色的了。不過(guò)，由于不斷出現(xiàn)氣流，把法埃與水汽分層排列，廣度不均，密度各異，加上高低空常有云層，不同程度地吸收并反射陽(yáng)光，我們這才看到各種奇異的色調(diào)斑剝陸離，諸多絢麗的色彩變化萬(wàn)千;任何人只要有幸將西方的景致一覽無(wú)余，只要有心觀看大自然不時(shí)展現(xiàn)的那一幅幅瞬息萬(wàn)變的彩畫(huà)，都會(huì)為之贊不絕口，喜不自勝。隨著夕陽(yáng)緩緩西墜，這種景觀也不斷變幻;尤其是在太陽(yáng)沉入地平線之后，由于角度更加適宜，五顏六色的光就都發(fā)射到地面上來(lái)。遇有些許云霧，更是如此。本來(lái)，只要太陽(yáng)還位于地平線之上，云霧便截住了不少夕陽(yáng)和色彩;而今太陽(yáng)從我們的視野消失，陽(yáng)關(guān)便更為直接地照射到密度各異的重重云靄與層層大氣的底部;一片嶄新的、更加燦爛的陽(yáng)光染紅了西天，一幅景觀色彩絢麗，變化萬(wàn)千，觀賞者固然賞心悅目，然而自嘆莫及。而我們之所以能領(lǐng)略如此無(wú)與倫比的美景，全應(yīng)歸功于——塵埃!
Alfred Russel Wallace
