Light Through a Narrow Slit; Shadows and the Color of Bodies
Rectilinear rays, the moon's changing brightness, and why shadows share a body's color
A page of the Treatise on Painting in Melzi's fair copy, opening with a proof that light passing through a long narrow slit and striking a columnar object of the same shape casts a shadow (e f) neither larger nor smaller than the slit at any distance, because all shadow-rays and luminous rays are rectilinear. A small inset diagram labels the slit, the object c d and the impact e f. A second section argues that shadows must always partake of the color of the shadowed body, and that nothing looks its true whiteness since its surroundings make it seem more or less white. Leonardo cites the moon, faint by day yet splendid by night, and notes the pupil dilates in the dark, seeing brighter light.
On this page
A long narrow slit casts a shadow equal to itself at any distance
When light passes through a long, narrow slit and strikes a shadowed object shaped and placed like the slit, the shadow takes the object's figure. Let the slit admitting light into the dark place be a b, the columnar object equal to the slit be c d, and the impact of its shadow-ray be e f. Leonardo says this shadow can be neither greater nor lesser than the slit at any distance, because all shadow and luminous rays are rectilinear.
Diagram of the slit and the columnar object
A small rectangular figure is set into the middle of the upper text. It marks the slit through which light enters, the columnar object c d matching the slit in shape, and the impact e f where its shadow-ray falls, illustrating the proof that the cast shadow equals the slit.
Shadows must partake of the color of the shadowed body
No thing appears in its natural whiteness, because the place where it is seen makes it look more or less white according as that place is darker or brighter. Leonardo draws the painter's lesson that shadow and appearance depend on surroundings and on comparison, which shows colors most perfectly when things differ most.
The moon as proof: brightness judged by contrast, and the widening pupil
The moon shows little brightness in the daytime sky, yet by night shines with such splendor that it gives back the image of the sun and of day, driving off the darkness. This arises from comparison and from the pupil being larger by night than by day, so that a wider pupil takes in a luminous body of greater quantity and more splendor.