f82v: those rainbows

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About double rainbows - Dante, Paradiso, Canto 12:

" As through a tenuous cloud two arcs curve
parallel and colored alike, when Juno commands
her handmaid,
the outer born from the inner one, like the
speech of that desirous nymph whom love
consumed as the sun does vapors
and cause people here to predict the
weather, thanks to the pact God made with
Noah, that the world will never again be flooded;
So the garlands of those two sempiternal
roses turned about us, so the outer replied
to the inner one;"

Dante refers to two things: Ovid, via the mention of the handmaid of Juno (Iris = rainbow) whom he associates with Echo (an echoed rainbow = a double rainbow), and the Bible's book of Genesis (8:13: "I have set my rainbow in the clouds, and it will be the sign of the covenant between me and the Earth)

Another interesting text about rainbows is the Ovide Moralisé, which was hugely popular in the 14th C.
It is in old French but here's my best translation of the parts which deal with rainbows:
XXI: Juno wanted to return to the sky. While she was climbing back up she was splashed and showered by the rainbow, which in latin is called Yris. And if any of you are wondering what Yris is, I will reply truthfully that it is a repercussion of the sun shining upon a cloud, and that it is shaped like a half compass, and that it drinks and draws water, not to retain it but to spill it... 
This part is followed by a long passage that details the virtues associated with rainbows: that it is a sign of God's covenant with Earth, that it is a symbol of charity and that it is a symbol of divine forgiveness, the light of god's grace reflected in the tears of the repenting sinner.