Magnifying grains of sand, seeds, and blades of grass:Optical effects in robert grosseteste’s de iride (on the rainbow) (circa 1228–1230)

In his treatise On the Rainbow (De iride), composed nearly four hundred years before the first known telescope, the English polymath Robert Grosseteste identified three striking optical effects: distant objects can be rendered close by; close-by large objects can be rendered small; and distant small objects can be rendered large. In the context of the history of optics, the first effect is especially striking. Grosseteste did not give details of the mechanisms underlying these effects but did mention the passage of rays through refraction in “diaphanous” or transparent bodies. While making no final claim that Grosseteste himself necessarily knew of or used lenses, this essay examines the coherence between the three optical effects described in Grosseteste’s treatise and two candidate proposals for the deployment of a single convex lens. A convex lens, deployed in different ways, is shown to produce all three of Grosseteste’s optical effects, in a manner strikingly aligned with the language that he uses to distinguish changes in the location and size of objects. The implications of this coherence for interpretations of On the Rainbow are discussed throughout the essay.