The colour of any object will appear more or less distinct in proportion to the extent of its surface. This proportion is proved, by observing that a face appears dark at a small distance, because, being composed of many small parts, it produces a great number of shadows; and the lights being the smallest part of it, are soonest lost to the sight, leaving only the shadows, which being in a greater quantity, the whole of the face appears dark, and the more so if that face has on the head, or at the back, something whiter.
Chap. CCLX.—A Precept relating to Painting.
Where the shadows terminate upon the lights, observe well what parts of them are lighter than the others, and where they are more or less softened and blended; but above all remember, that young people have no sharp shadings: their flesh is transparent, something like what we observe when we put our hand between the sun and eyes; it appears reddish, and of a transparent brightness. If you wish to know what kind of shadow will suit the flesh colour you are painting, place one of your fingers close to your picture, so as to cast a shadow upon it, and according as you wish it either lighter or darker, put it nearer or farther from it, and imitate it.
Chap. CCLXI.—Of Colours in Shadow.
It happens very often that the shadows of an opake body do not retain the same colour as the lights. Sometimes they will be greenish, while the lights are reddish, although this opake body be all over of one uniform colour. This happens when the light falls upon the object (we will suppose from the East), and tinges that side with its own colour. In the West we will suppose another opake body of a colour different from the first, but receiving the same light. This last will reflect its colour towards the East, and strike the first with its rays on the opposite side, where they will be stopped, and remain with their full colour and brightness. We often see a white object with red lights, and the shades of a blueish cast; this we observe particularly in mountains covered with snow, at sun-set, when the effulgence of its rays makes the horizon appear all on fire.
Chap. CCLXII.—Of the Choice of Lights.
Whatever object you intend to represent is to be supposed situated in a particular light, and that entirely of your own choosing. If you imagine such objects to be in the country, and the sun be overcast, they will be surrounded by a great quantity of general light. If the sun strikes upon those objects, then the shadows will be very dark, in proportion to the lights, and will be determined and sharp; the primitive as well as the secondary ones. These shadows will vary from the lights in colour, because on that side the object receives a reflected light hue from the azure of the air, which tinges that part; and this is particularly observable in white objects. That side which receives the light from the sun, participates also of the colour of that. This may be particularly observed in the evening, when the sun is setting between the clouds, which it reddens; those clouds being tinged with the colour of the body illuminating them, the red colour of the clouds, with that of the sun, casts a hue on those parts which receive the light from them. On the contrary, those parts which are not turned towards that side of the sky, remain of the colour of the air, so that the former and the latter are of two different colours. This we must not lose sight of, that, knowing the cause of those lights and shades, it be made apparent in the effect, or else the work will be false and absurd. But if a figure be situated within a house, and seen from without, such figure will have its shadows very soft; and if the beholder stands in the line of the light, it will acquire grace, and do credit to the painter, as it will have great relief in the lights, and soft and well-blended shadows, particularly in those parts where the inside of the room appears less obscure, because there the shadows are almost imperceptible: the cause of which we shall explain in its proper place.