Light, Shadow, and the Appearance of Relief
How light spreads through air, why an object lit head-on shows no relief, and the quality of lights
This folio pursues the physics of light and shade. Diagrams at the top trace rays reaching two eyes from round bodies, some seen as distinct and some indistinct, and a brief note holds that the apparent enlargement of a luminous body comes from the air its light traverses. A worked example (with body a, light c, incident lines c m and c n, and eye at b) argues that an opaque body seen along the line of incidence shows no relief, since it is uniformly lit and the eye can detect no difference of shadow. A lower diagram of light passing through an aperture e onto nearer wall a b and farther wall c d gives a rule on the "quality of lights" and its usefulness for engravers of delicate work.
On this page
Light to two eyes: distinct and indistinct bodies
Three diagrams at the top margin trace rays received by two eyes from round bodies, some illuminated and some shaded. From right to left they are labeled a – b, distinct, and indistinct.
The amplification of a luminous body
In notably smaller script, a single proposition. The amplification of a luminous body derives from the air that its light traverses.
An opaque body lit along the line of incidence shows no relief
Let a be the opaque body, c the light, c m and c n the incident luminous lines, and b the eye. Because light c falls on all of part m n, the relief there is wholly illuminated; the eye at b sees no difference of shadow and light, so every part appears one colour and the prominent, rounded parts do not show.
Diagram: eye b, object a, light c
An eye b observes a bumpy object a, with visual rays entering from m and n at the object's edges. A light source c illuminates it from behind the eye. The labels read m – a b c – n.
Quality of lights: intensity, distance, and engraving
Light from the horizon f g enters through aperture e to strike nearer wall a b and farther wall c d. As many times as a b enters into c d, so many times brighter it is; likewise for point e. Such light is good for those who engrave delicate works.