Placing Shadows and Lights on a Faceted Body
Three kinds of light, and how the angle of incidence governs a face's brightness
The page closes a discussion of three kinds of illumination for the painter: the mixed particular-and-universal light of porticoes, and the particular light of a room lit by a single window, with attention to reflections passing through narrow spaces between neighbouring bodies. A new chapter [755] opens the 'Rule for placing the due shadows and lights on a figure or faceted body,' stating that the darkness or brightness on each face is proportional to the size of the angle between the luminous body's central ray and the illuminated face. The rule is introduced through the example of an octagonal columnar body. The text is written in Melzi's clean compiler hand with no diagram on this page.
On this page
Three sorts of light: porticoes, single-window rooms, and reflections
The lights of porticoes are mixed of particular and universal light, while dwellings lit from a single window give particular light. For the first sort the lights must occupy large fields, and one must also account for light reflected from one body onto another where it enters through narrow spaces between bodies lit by universal light.
[755] Brightness of a face set by the angle at its centre
The greater or lesser darkness of shadow, or brightness of light, striking the faces of a faceted body is as the greater or lesser size of the angle enclosed between the central line of the luminous source (which strikes the middle of the lit side) and the surface of that side. The rule is posed for a body shaped like an octagonal column.