The Roman History · Amm. 20.11.26

To these portents were added continual rainbows. A short explanation will serve to show how these appearances are formed. The vapours of the earth becoming warmer, and the watery particles gathering in clouds, and thence being dispersed in spray, and made brilliant by the fusion of rays, turn upwards towards the fiery orb of the sun and form a rainbow, which sweeps round with a large curve because it is spread over our world, which physical investigations place on the moiety of a sphere.

Ammianus Marcellinus · translated by C. D. Yonge · 1911 reprint
C. D. Yonge translation (preface dated February 1862); G. Bell and Sons, 1911; Project Gutenberg #28587
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