Of luster: highlights on shaded bodies and dark grounds
Lit surfaces act as confused mirrors of sun and sky; luster strongest on black
This leaf finishes a discussion of illuminated surfaces, noting that the lit part of a shadowed body looks less bright the closer it lies to its own highlight, and that such surfaces behave like confused mirrors that vaguely gather the image of the sun, sky, a window's light and a wall's darkness. A new section headed DE LUSTRO (Of Luster) then opens with chapters 771 and 772, which hold that a highlight stands out most from its ground when generated on a blacker surface, since polished surfaces act like mirrors and the sun shows more powerfully against a dark ground. The page is unbroken text with no drawing.
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Polished lit surfaces behave like confused mirrors
The illuminated part of a shadowed body looks less luminous the nearer it comes to its own highlight, because of the strong contrast at their border. Such surfaces are like confused mirrors that dimly take in the image of the sun, the sky that forms their ground, the light of a window and the darkness of the wall around it.
A highlight tells strongest against a black ground
Under the heading DE LUSTRO, chapter 771 holds that among equally polished bodies the luster generated on the blacker surface differs most from its ground, since polished surfaces act as mirrors. Chapter 772 adds that among lusters of equal power the one on the darker ground shows the most excellent brightness.