Edges of Light and Shade; the Highlight That Moves with the Eye
The brightest and darkest boundaries depend on the ground behind them, and the highlight shifts as the viewer moves
The page explains how the apparent strength of a lit or shaded edge depends on the ground it meets: a lit edge looks brightest against a dark ground (at d), but against a bright ground its peak brightness withdraws to between the ground f and the shadow, while a shaded edge is darkest against a bright place (l) and, against a dark ground, keeps its deepest darkness between that boundary and the light at o. A following chapter treats the highlight (lustro) that turns and shifts as the viewer's eye moves: on a round body lit from point a and seen from d, the highlight appears at c, and moves from c toward n as the eye moves from d toward a. Two small circle diagrams of illuminated spheres accompany the text. The page ends with the heading of a chapter on where object-cast shadows should end.
On this page
A lit edge is brightest against a dark ground
The part of a light that appears brightest is the one that ends against the dark, at d; but where the illuminated part borders a bright ground its edge looks less bright, and its greatest clarity appears between the boundary of the ground f and the shadow.
A shaded edge is darkest against a bright ground
The boundary of a shadowed body set against a bright place (l) appears much darker than the rest; if instead the shadow ends against a dark ground its edge looks brighter than before, and its deepest darkness falls between that boundary and the light, at the point o.
The moving highlight (lustro) on a round body
On the round body figured here, lit by the point a with the illuminated part b c and the eye at d, the highlight appears at point c. As much as the eye shifts from d toward a, so much the highlight shifts from c toward n.