Eastlake's Methods and Materials of Painting of the Great Schools and Masters
Eastlake, Sir Charles Lock [One-time President of the Royal Academy], Methods and Materials of Painting of the Great Schools and Masters [Formerly titled: Materials for a History of Oil Painting]. Vol. One. New York; Dover Publications, Inc. 1960 [Originally published by Longman, Brown, Green and Longmans in 1847]
A man's head of the ordinary complexion seen at a certain distance in the unpronounced light and shade of the open air, or of a room with more than one window, or with a diffused light, exhibits that appearance which Leonardo da Vinci somewhere describes, and which is common in Venetian pictures. The effect of the minuter shades or dark [p. 355] colours [of eyes, brows, beard, &c.] is, as Leonardo observes, to colour the whole mass--to make it darker and warmer. The darker side of the face [the light being assumed to predominate on one side] has, seen at such a distance and under such circumstances, a browner hue only, and is hardly distinguishable from dark local colour on the light side. Barry [the painter] somewhere describes the shadow of Titian's flesh as "flesh colour darkened and embrowned only." There is, however, a fine gradation both in light and dark, [more perhaps in light]; the retiring parts of the face, as for instance the side of the cheek and temple, without losing their broad warmth of colour, are less illumined than the cheekbone, and the forehead is often the same; the nose again, even in fair subjects, looks darker, partly because surrounded with darks, and partly because its minute lights [at the point and on the bridge] become invisible, as Leonardo truly observes, at a little distance. There is also no shine on dark hair at a certain distance.
This distant, broad, shadowy effect is most agreeable when the surface in panting is not too transparent and glossy, but rather mealy. This effect is produced by using, where possible, semi-opaque colours [always darker than the colour on which they are scumbled] in tinting, toning, and darkening. The same appearance is to be aimed at in golden complexions; they should not look too glassy and glossy, but have a due opacity. This may be extended [ p. 357] even to half-shadows. In this is seen one great difference between the Venetian and Flemish masters; the Flemish painters can never have too much transparency, and they certainly manage it well; but the Venetians with equal, or with scarcely less splendour, have more solidity, and yet their system, in its shadowy breadth, agrees more with ideal and somewhat distantly seen forms. [pp. 355-357]