A Treatise on Painting, by Leonardo da Vinci
Senex and Taylor, London
The Surfaces of Dense Bodies are incompass'd with Lights and Shadows of very different Qualities. Of Lights there are two kinds; the one Original, the other borrowed: Original Light, is that inherent in any Body, and which it does not receive from any other; as Fire, the Sun, and even the Air, which last, however, though it be well stored with Light, yet does it in effect receive it all from the Sun: Borrowd Light, is reflected Light; that which a Body has not in it self, but receives from another. To come to the Purpose then; There can be no Reflex of Light from that side of a Body on which it is shadow'd; that is, from that side turn'd towards any dark Body, or Place, as Thickets, Trees, Shrubs, Herbs, and the like; For though each Leaf and Branch, receives the Light towards which it is turned, yet does the great Quantity of Leaves and Branches, form an Opake Body, which the Light cannot penetrate.