"Reproduction of Millī MS 867 fol. 7r, showing his discovery of the law of refraction (from Rashed, 1990). The lower part of the figure shows a representation of a plano-convex lens (at the right) and its principal axis (the intersecting horizontal line). The curvature of the convex part of the lens brings all rays parallel to the horizontal axis (and approaching the lens from the right) to a focal point on the axis at the left."