A Treatise on Painting, by Leonardo da Vinci
Senex and Taylor, London
In representing of Bodies, you must always give them such Lights, as are most suitable to the Places they are supposed to be in. For Instance, if they be suppos'd in the Country, and in the open Air, the Sun being hidden, they ought to be incompass'd with an almost Universal Light; if the Sun be seen, the Shadows must be very dark with respect to the other Parts which receive the Light; and all the Shadows both Primitive and Derivative, must have their Extremities bold, and defined: The Light accompanying these Shadows, must be extremely faint; because the Air, to whose Reflexion they owe that little Light they receive, communicates at the same time its own Colour; weakening the Light it conveys, by mingling its own Azure along with it. This is easily observable in White Objects; such Parts of which, as are illumined by the Sun, plainly appearing tinged with the Colour of that Luminary; but discovers it self still more evidently, when the Sun, hidden behind a Cloud, illumines it with his Rays, and makes it appear Red and Inflamed: For then all Bodies receiving Light from the Cloud, will be tinged and coloured with its Redness; while the other sides of the Bodies, turned from the Cloud will appear obscure, and tinged with the Azure of the Air; so that a Person, observing this Object thus differently illustrated, will Imagine it of two Colours. 'Tis a certain Maxim then, founded upon what we know of Nature, and the Cause of these Lights, and Shadows, that to represent them aright, they must participate of that which produced them; and that unless we make them retain something of their first Cause, our Imitation of Nature will be Lame and Imperfect. But if the Object you represent, be supposed in a Chamber a little illumined, and that you view it from without, standing in a Line with the Light that breaks in upon it, the Shadows of that Figure must of Necessity be very soft, and the Figure cannot fail, of being very graceful, and of doing Credit to the Painter; for the*Relievo will be bold, notwithstanding the softness of the Shadows; and these will be the more eminently so, on that side of the Chamber which is the most enlightened, the Shadows there being almost insensible: The Reason of which shall be delivered hereafter.
* The Relievo is an embossed Figure in Sculpture; in Painting it is used for that part which comes boldly out, as if it were really embossed.