547.) Others therefore fabled that the bow hath a head like a bull, by which it swallows up rivers.
But what is the cause of the rainbow?
It is evident that what apparent things we see come to our eyes in right or in crooked lines, or by refraction: these are incorporeal and to sense obscure, but to reason they are obvious.

Those which are seen in right lines are those which we see through the air or horn or transparent stones, for all the parts of these things are very fine and tenuous; but those which appear in crooked lines are in water, the thickness of the water presenting them bended to our sight.

This is the reason that oars in themselves straight, when put into the sea, appear to us crooked.

The third manner of our seeing is by refraction, and this is perspicuous in mirrors.
After this third sort the rainbow is affected.

We conceive it is a moist exhalation converted into a cloud, and in a short space it is dissolved into small and moist drops.