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 adirection 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 reasonof this is that near the horizon we look through a very great thickness of thelower atmosphere，which is full of the larger dust particles reflecting whitelight，and this diluter the pure blue of the higher atmosphere seen beyond，And inthe vicinity of the sun a good deal of the blue light is reflected back intospace by the finer dust，thus giving a yellowish tinge to that which reaches usreflected chiefly from the coarse dust of the lower atmosphere. At sunset andsunrise，however，this last effect is greatly intensified，owing to the greatthickness of the strata of air through which the light reaches us. The enormousamount of this dust is well shown by the fact that then only we can look full atthe sun，even when the whole sky is free from clouds and there is no apparentmist.
But the sun’s rays then reach us after having passed，first，throughan enormous thickness of the higher strata of the air，the minute dust of whichreflects most of the higher strata of the air，the minute dust of which reflectsmost of the blue rays away from us，leaving the complementary yellow light topass on，Then，the somewhat coarser dust reflects the green rays，leaving a moreorange-coloured light to pass on；and finally some of the yellow isreflected，leaving almost pure red. But owing to the constant presence of aircurrents，arranging both the dust and vapour in strata of varying extent anddensity，and of high or low clouds which both absorb and reflect the light invarying degrees，we see produced all those wondrous combinations of tints andthose gorgeous ever-changing colours which are a constant source of admirationand delight to all who have the advantage of an uninterrupted view to the westand who are accustomed to watch for those not infrequent exhibitions of nature‘skaleidoscopic colour painting. With every change in the altitude of the sun thedisplay changes its character；and most of all when it has sunk below thehorizon，and owing to the more favourable angles a larger quantity of thecoloured light is reflected toward us，Especially when there is a certain amountof cloud is this the case. These，so long as the sun was above thehorizon，intercepted much of the light and colour，but when the great luminary haspassed away from our direct vision，his light shines more directly on the undersides of all the clouds and air strata of different densities；a new and morebrilliant light flushes the western sky，and a display of gorgeous ever-changingtints occurs which are at once the delight of the beholder and the despair ofthe artist. And all this unsurpassable glory we oweto——dust！