Res Gestae
Ammianus Marcellinus
Ammianus Marcellinus. Ammianus Marcellinus, with an English translation, Vols. I-III. Rolfe, John C., translator. Cambridge, MA; London: Harvard University Press; W. Heinemann, 1935-1940 (printing).
For when the fighting slackened, wet weather followed, dripping clouds with menacing darkness appeared, and the ground was so drenched with continual rains, that soft and sticky mud caused general trouble in that region of rich turf. And, besides all this, thunder and lightning with repeated crashes terrified the timorous minds of men.
More than this, rainbows were constantly seen; and how that phenomenon is wont to occur, a brief explanation will show. The warmer exhalations of the earth and its moist vapours are condensed into clouds; these are then dissipated into a fine spray, which, made brilliant by the sun’s rays that fall upon it, rises swiftly and, coming opposite the fiery orb itself, forms the rainbow. And the bow is rounded into a great curve, because it extends over our world, which the science of natural philosophy tells us rests upon a hemisphere.[*](The meaning seems to be that the vault of the heavens is therefore a hemisphere.)
Its first colour, so far as mortal eye can discern, is yellow, the second