Light Striking a Head: Rays and Angles of Illumination
A profile head lit from a single source, with lettered rays and a numbered scale of lines
This page joins a short passage on the illumination of a head to a large drawing of a man's profile struck by rays from a single light source at the upper left. The text states that the part of the head receiving the ray at more nearly equal angles is the more strongly illuminated, while parts meeting the rays at less equal angles are less luminous, the light acting in the manner of a blow. A fan of straight rays labelled from the source down across the face is keyed to a lettered legend at the left (lines B through M against the numbers 1 to 11), which is not included in the supplied transcription. Further untranscribed writing is faintly visible behind the drawing.
On this page
More equal angles of incidence give brighter light
If a is the light and the head the body it illuminates, the part of the head that receives the ray at more nearly equal angles is the more strongly lit, while the part receiving the rays at less equal angles is less luminous. The light in its action is likened to a blow (colpo).
Profile head crossed by a fan of light rays
A large drawing shows a man's head in right profile, the brow, nose and lips finely modelled. From a single point at the upper left a fan of straight ruled lines spreads across the face, marking successive rays of light striking the features. Lettered marks (including B, D, E, F, G) run along the forehead and face.
Lettered scale of the rays at the left margin
A column at the left lists a sequence of letters against numbers, the lines B, C, D, E, F, G, H, I, L, M keyed to the values 1 through 11, evidently a scale relating each ruled ray to the head it strikes. This legend is not part of the supplied transcription.