Fifth Book on Light and Shade.
Principles of reflection (203. 204).
203.
Of the Way in Which the Shadows Cast by Objects Ought to Be Defined.
If the object is the mountain here figured, and the light is at the point a, I say that from b d and also from c f there will be no light but from reflected rays. And this results from the fact that rays of light can only act in straight lines; and the same is the case with the secondary or reflected rays.
204.
The edges of the derived shadow are defined by the hues of the illuminated objects surrounding the luminous body which produces the shadow.
On reverberation.
205.