The relations of luminous to illuminated bodies.

The middle of the light and shade on an object in light and shade is
opposite to the middle of the primary light. All light and shadow
expresses itself in pyramidal lines. The middle of the shadow on any
object must necessarily be opposite the middle of its light, with a
direct line passing through the centre of the body. The middle of
the light will be at _a_, that of the shadow at _b_. [Again, in
bodies shown in light and shade the middle of each must coincide
with the centre of the body, and a straight line will pass through
both and through that centre.]

[Footnote: In the original MS., at the spot marked _a_ of the first
diagram Leonardo wrote _primitiuo_, and at the spot marked
_c_--_primitiva_ (primary); at the spot marked _b_ he wrote
_dirivatiuo_ and at _d deriuatiua_ (derived).]

Experiments on the relation of light and shadow within a room
(137--140).