As to the variegation in colour, it is due simply to its double source, derived partly from the sun, partly from the moist cloud. The moisture produces lines now blue, now green, now purple-like, and orange or red--the two shades, dull and bright, combining to produce this diversity. So also, a purple garment does not always come out 14 in exactly the same tint from the same dye. Differences depend upon the length of time it has been steeped, the consistency and the amount of moisture in the dye it has imbibed: it may be dipped and boiled more than once, or it may have received only one immersion. In like manner then, when there are the two elements, sun and cloud, in other words, object and mirror, it is little wonder that as many varieties of colour are generated as can be produced from them in higher or lower tone in countless different categories. For example, there is one 15 colour that proceeds from the light of fire, another from a light that is duller and less violent than fire. In other details concerning the rainbow the method of inquiry is full of uncertainty; there is nothing concrete to lay hold upon, and conjecture must be ventured in every direction. But in this question of its origin doubt is precluded; for it is evident that the causes of the rainbow are two in number, sun and cloud. The bow never appears when the sky is clear, and never when it is so cloudy as to hide the sun. It must, therefore, unquestionably arise from these, failing either of which it cannot come into being. IV