Robert Grosseteste

On the Rainbow

Introduction by Neil Lewis

Grosseteste says: "In general, then, there will be four translucent [media] through which the sun's ray penetrates, namely pure air containing mist, secondly the mist itself, thirdly the highest and most rarefied part of the dew coming from mist, and fourthly the lower and denser part of that same dew. Then it is necessary from the things which were said earlier about the refraction of the ray and the size of the angle of the refraction at the boundary of the two translucent [media], that the solar rays are first broken at the boundary of air and mist, and then at the boundary of mist and dew, so that the rays connect through these refractions in the density of the dew, and after they are again broken as though by a pyramidal cone, they pour out not in the rounded pyramid of the second [stage], but in a form similar to the curved surface of a rounded pyramid, a form which is opened up opposite to the sun. And therefore its shape is that of a bow, and where we are the rainbow appears in the south."

Original Latin