The rainbow is a phenomenon produced by a cloud which is hollow and dewy, and which is disposed toward rain in an infinite fall of droplets, shining as though in a mirror; it has a bow-shaped form, emitting various colors from the rays of the sun. Alternatively, it is produced by the moon, though the way it is caused by the rays of the moon, [occurs] but twice every fifty years, as Aristotle says. And so the bow in the sky is a watery cloud, penetrated through all of its parts by the rays of the sun, as [Marc. ??] says. It is called a “bow” from its similarity to a curved bow, since it extends into a kind of convexity in the very top of the clouds, and it bends and extends its tips back down to the earth, as Isidore says.

original latin

Nam in Iride, propter eius transparentiam et dyaphaneitatem, refulgent species et formae sibi oppositae et obiectae, et ideo arcus apparet coloratus, nam secundum Bedam colorem contrahit ex quatuor elementis, nam in ipso velut in speculo formae et species elementorum relucent. Unde ex igne contrahit colorem rubeum in supremo, ex terra viridem in infimo, ex aere vero Hyacinthinum, et ex aqua caeruleum in medio, ut dicit Beda. Et hi colores sunt ad invicem coordinati secundum Aristoteles in libris Meteororum. Primo enim est color rubeus seu vinosus, qui sit ex radio tangente superficiem rotunditatis nubis, deinde sequitur color mixtus, scilicet blaveus vel caeruleus, secundum qualitatem vincentem et dominantem in vapore existente in medio nubis, deinde apparet in infimo color viridis, in inferiori parte nubis, ubi vapor magis est terrestris, et hi colores sunt principaliores aliis, quia secundum Philosophum multos alios habet colores, inter quos distinguere sensui est difficile, esicut ideo dicit Philosophus, quod nullus pictor potest depingere colores arcus sive fingere....

...Quo ad virtutem vero eius sive operationem sui interpositione fervorem Solis temperat et mitigat, per sui resolutionem in pluviam vel in rorem inferiora vegetat et foecundat, per suam figuram et multiformem colorum variationem superiora pulcrificat et decorat, dominium humiditatis in aere signat.

Sicut etiam Solis facit indicium, quia in qua parte caeli sit Sol sua praesentia indicat, quia nunquam est in eadem parte caeli cum Sole, sed semper ex adverso. Sole enim existente in oriente, apparet arcus in occidente, et econverso, Sole existente in meridie, apparet arcus in septentrione. In parte autem australi sive meridionali nunquam apparet, sicut dicit Beda.

In hora autem recte meridiana nunquam apparet, cuius rationem assignat Philosophus, quia in meridie, quando mediat Sol orbem, tunc resplendet radius eius in utroque horizonte, secundum aequalitatem, quare tunc non plus resplendet in una parte quam in alia, et ideo non apparet arcus in aliqua parte caeli in puncto meridiei, ut dicit Aristoteles. Rarissime autem videtur de nocte, immo nunquam nisi in plenilunio, ut dicit Beda. et hoc non contingit nisi bis in 50. annis secundum Aristotelem.