Grosseteste's contribution was to emphasize the importance of falsification in the search for true causes and to develop the method of verification and falsification into a systematic method of experimental procedure.
Grosseteste was the first medieval writer to break with the ancient tradition of discounting refraction in the rainbow. In another way, however, Grosseteste is firmly bound to his predecessors' rainbows an experimentalist in principle, in practice he strays very little from the Aristotelian tradition of selective observation constrained by geometry. Furthermore, Grosseteste's rainbow theory often appeals to authority, leading historian Bruce Eastwood to remark: "When sources fail, he invents, and the invention is never contradictory to literary sources." (In fairness, Grosseteste made several observations about the rainbow that gainsaid ancient authority.)
Grosseteste was deeply concerned with the detailed investigation of natural phenomena. It was the inspiration of this attitude of mind, together with Grosseteste's emphasis on the importance of mathematics, that was perhaps his chief legacy to thinkers in fourteenth-century Oxford who were developing the beginnings of a mathematical physics.
Lux was probably Grosseteste's ingenious substitute for the immaterial pneuma of the Neoplatonists and the 'animal spirits' of medical writers.
Grosseteste's experimental method was quite different from a method of controlled experiment. Grosseteste made no use of such a method in his writings, deriving his conclusions on the basis of a mix of considerations, appealing to authority and everyday observation (the Latin “experimentum”). He made use of thought experiments and certain metaphysical assumptions, such as the assumption of a principle of “least action.”