Res Gestae

Its first colour, so far as mortal eye can discern, is yellow, the second

It shows this combination of beautiful colours, as earthborn minds conceive, for the reason that its first part, corresponding in colour with the surrounding air, appears paler; the second is tawny, that is, somewhat more vivid than yellow; the third is red, because it is exposed to the brightness of the sun, and in proportion to alternation in the air absorbs its brilliance most purely, being just opposite;[*](I.e. the air is so affected by contact with the first two bands that it becomes more receptive of the effect of the sun’s rays.) the fourth is violet, because receiving the brightness of the sun’s rays with a thick rain of spray glittering between, through which it rises, it shows an appearance more like fire; and that colour, the more it spreads, passes over into blue and green.