Shadow is diminution of light.

Darkness is absence of light.

Shadow is divided into two kinds, of which the first is called
primary shadow, the second is derived shadow. The primary shadow is
always the basis of the derived shadow.

The edges of the derived shadow are straight lines.

[Footnote: The theory of the _ombra_ dirivativa_--a technical
expression for which there is no precise English equivalent is
elaborately treated by Leonardo. But both text and diagrams (as Pl.
IV, 1-3 and Pl. V) must at once convince the student that the
distinction he makes between _ombra primitiva_ and _ombra
dirivativa_ is not merely justifiable but scientific. _Ombra
dirivativa_ is by no means a mere abstract idea. This is easily
proved by repeating the experiment made by Leonardo, and by filling
with smoke the room in which the existence of the _ombra dirivativa_
is investigated, when the shadow becomes visible. Nor is it
difficult to perceive how much of Leonardo's teaching depended on
this theory. The recognised, but extremely complicated science of
cast shadows--_percussione dell' ombre dirivative_ as Leonardo
calls them--is thus rendered more intelligible if not actually
simpler, and we must assume this theory as our chief guide through
the investigations which follow.]

The darkness of the derived shadow diminishes in proportion as it is
remote from the primary shadow.

Different sorts of derived shadows (160-162).