A Treatise on Painting/Chapter 211
Chap. CCXI.—Of Objects placed on a light Ground, and why such a Practice is useful in Painting.
When a darkish body terminates upon a light ground, it will appear detached from that ground; because all opake bodies of a curved surface are not only dark on that side which receives no light, and consequently very different from the ground; but even that side of the curved surface which is enlightened, will not carry its principal light to the extremities, but have between the ground and the principal light a certain demi-tint, darker than either the ground or that light.