A Body Lit by the Sky's Hemisphere, and Why It Casts No Reflection
Points k, a, b lit by the arc h c i above a shadowed earth
A page of Book Five of the Treatise on Painting, headed QVINTA and numbered 229, analysing how a body is illuminated and shadowed by the luminous hemisphere of the sky above a dark earth. A pen diagram of a semicircle lettered h, c, i, with interior points k, a, b and a base line marked r, d, e, f beneath a small circle, shows why k is brighter than a and b according to how much of the bright hemisphere and dark earth each point sees. The text concludes that such a body can bear no reflected light, since reflection comes only after the principal shadow, which lies where the body touches the ground. It ends with the chapter title 'How reflection is generated in universal lights.'
On this page
A point lit by the whole hemisphere, not by the dark earth
The point k is seen by the entire hemisphere h c i and by no part of the earth's darkness, so it is more illuminated than a, which sees only the arc c d. The point b is darker still, seeing only e d, half of c d, together with all the earth-shadow r d and the darker part r f, where the light of the arc e c is lacking.
Why this body can have no reflected light
Reflected light falls only after a body's principal shadow. Here the principal shadow lies at the point where the body touches the plane of the earth, for there it is wholly deprived of light, so no reflection can arise.
Diagram of the illuminating hemisphere
A semicircle drawn at mid-page stands for the luminous hemisphere of sky, its arc lettered h, c, i, with interior points k, a and b and a base line marked r, d, e, f beneath a small circle representing the lit body standing on the ground.