Abstract

This article presents a study and a critical edition of the short anonymous treatise on the rainbow starting with the incipit Inter omnes impressiones. The text was known to Nicole Oresme who engages with it twice: in his Questiones in Meteorologica de prima lectura and in Le livre du ciel et du monde. This Tractatus de iride, previously unknown to scholars, is transmitted in three late thirteenth-century manuscripts. It uses Robert Grosseteste’s theories of the rainbow as caused by the refraction of sunlight and of colour as light incorporated in aereal particles. However, contrary to Grosseteste, the Tractatus de iride adopts the idea of different degrees of incorporation of light, which is also found in the scientific writings by Adam of Exeter, a Franciscan scholar belonging to the same Oxonian circle as Grosseteste. Moreover, the Tractatus de iride develops original propositions in regard to the role of individual raindrops, the importance of the angle from which the rainbow is observed, and the idea of the spirituality of the colours in the medium, which were central also for Roger Bacon’s and Nicole Oresme’s own theories of the rainbow.