eastern boundary of Tottenham. Its colour was white, cloudy, or greyish, but a part of its western limb seemed to exhibit tints of a faint sickly green. After some time the moon became darkened by clouds, and the rainbow of course vanished.”
The brilliant colours of the solar rainbow are
frequently produced by the clouds without any prismatic arrangement. The light of the sun is decomposed by a process called absorption: for example, white light is composed of red, yellow, and blue rays, in certain proportions; now, if in passing through, or falling upon any substance whatever, the red rays are stifled or absorbed, while the yellow and blue are allowed to pass or to be reflected, it is obvious that such a substance cannot appear white, because one of the elements of white light, namely, the red, is wanting; it must therefore appear of such a colour as results from the combination of yellow and blue; the substance will therefore appear green. So, also, when white light falls upon what we call a red surface, the yellow and blue rays are stifled or absorbed, leaving the red only to be reflected. Now, when we consider the various ways in which this absorption may take place; one or two, or all of the coloured rays being absorbed in every possible proportion, it is easy to form some idea of the manner by which the innumerable tints of the sky are produced.
It has been calculated, that, of the horizontal sunbeams which pass through two hundred miles
of air, scarcely a two thousandth part reaches the earth. A densely formed cloud must therefore detain a much larger share; and those dark and sombre forms, which sometimes make the sky so gloomy, can only result from the abundant absorption of the solar light. The brilliant whiteness which their edges occasionally exhibit, must result from the more copious transmission of light, so that the depths of shade in a cloud may be regarded as comparative measures of the varied thickness of its mass.
Sometimes the clouds absorb equally all the solar rays, in which case the sun and moon appear through them perfectly white. Instances are recorded in which the sun appeared of a pale blue. It has also been observed to be orange at its upper part, while the lower was of a brilliant red.
The position from which clouds are seen, has much to do with their colours; and it seems difficult sometimes to believe that the clouds, which in the evening are seen drenched with crimson and gold, are the same we beheld absolutely colourless in the middle of the day.
In the immediate neighbourhood of the sun the
most brilliant colours may be disclosed; and their vividness and intensity diminish, and at last disappear at some distance from it. Parry noticed some white fleecy clouds, which, at the distance of fifteen or twenty degrees from the sun, reflected from their edges the most soft and tender tints of yellow, bluish green, and lake; and as the clouds advanced the colours increased gradually, until they reached a sort of limit two degrees below the solar orb. As the current continued to transport them, the vividness of colour became weakened by almost insensible degrees until the whole assemblage of tints vanished.