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FIG. 107. Diagrams from the thirteenth/century MS., Royal 7. F. viii, the
British Museum, illustrating Roger Bacon's classification ofthe properties ofcurved
refracting surfaces, in the Opus Majus,
v. Rays go from each end of the
object (res, r),
are bent at the curved surface
separating the
optically
rarer
(sukilior, s) and denser
(fansw, J) media, for example air and glass, and meet at the
eye (owlus, 0). The
image (ywgo, y) is seen on a projection of these bent rays entering the eye, and is
magnified or diminished according to whether the concave (i~iv) or convex (v-viii)
surface is towards the eye, whether the eye is on the rarer (i, ii, v, vi) or denser (iii,
iv, vii, viii) side ofthe curvature, and whether the eye is on the side of the centre of
IV