I shall draw, therefore, a figure in which all these matters are made clear as far as possible on a surface, but the full demonstration would require a body like the eye in all the particulars aforesaid. The eye of a cow, pig, and other animals can be used for illustration, if anyone wishes to experiment.
v. i. iii. 3, ed. Bridges as quoted in A.C. Crombie, Robert Grossetest and the Origins of Experimental Science 1100-1700 (1953) - Opus Majus, c. 1267