The Ordered Universe project is pleased to announce its latest publication, for the Applied Optics journal, Vol. 56 (2017), G197-G204, and fully open access. The experiments and writing of the paper were led and marshalled by expertly Joshua Harvey (Mellon Foundation funded graduate student at Oxford University, Dept of Engineering Science and Pembroke college), with assistance from other members of the team, from the sciences and humanities. The paper focuses on the middle part of Grosseteste’s treatise On the Rainbow (De iride) and the shape that the rainbow forms in the sky. This precedes discussion of the colours of the rainbow, covered in other Ordered Universe papers. The current paper offers an historical context for the treatise before moving to the main discussion, testing Grosseteste’s optical thought with physical experiment and physics-based simulation. The results are available below, and show, again, the benefits of collaborative working to unlock problems posed by thinkers of the past.