 Bacon’s own important contribution is to be found in his calculation of the measured value of 42 degrees for the maximum elevation of the rainbow. This was probably done with an astrolabe, and in this, Bacon advocates the skillful mathematical use of instruments for an experimental science. That Bacon had mathematical competence in this field can be seen in his account of the halo in the Opus tertiium (OQHI) and in his complex arguments in De speculis comburentibus. Bacon takes up Grosseteste’s theory of refraction and tries to work out the difficulties in the latter. It would appear at first glance that Bacon is rejecting the notion of refraction out of hand (Lindberg 1966), but in fact he is only demonstrating that refraction as formulated by Grosseteste did not make experimental or rational sense. Bacon knew the rules governing the geometry of refraction. Important here is his emphasis on the role of individual drops of water for the process of reflection and refraction. The new research on the De iride of Grosseteste by Cecilia Panti and the other editors of the Oxford/Durham Robert Grosseteste project will shed much light on these matters. A correct account of the rainbow would be summarized some forty years after Bacon in the De iride of Theodoric of Freiberg (d. c. 1307) [TFT], with his account of a single reflection and double refraction in single drops of water. 