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！ |
大學英語---