Chapter 10: On the rainbow and the causes of clouds

1. The rainbow is so called from its resemblance to a bent bow. Its proper name is Iris and it is called Iris, as it were _aeris_ (of the air), because it comes down through the air to earth. It comes from the radiance of the sun when hollow clouds receive the sun’s ray full in front, and they create the appearance of a bow, and rarified water, bright air, and a misty cloud under the beams of the sun create those varied hues.

2. Rains (_pluviae_) are so called because they flow, as if _fluviae_. They arise by exhalation from earth and sea, and being carried aloft they fall in drops on the lands, being acted upon by the heat of the sun or condensed by strong winds.

13. Shadow (_umbra_) is air that lacks sun, and is so called because it is made when we interpose ourselves in the rays of the sun. It moves and is ill-defined, because of the motion of the sun and the force of the wind. As often as we move in the sun, it seems to move with us, because wherever we encounter the rays of the sun, we take the light from that place, and so the shadow seems to walk with us and to imitate our motions.