Towers fading in fog; shadow-spots on distant faces
How depths of mist darken tower-tops, and how sky-light models the parts of the face
Two diagrams support a discussion of how atmosphere and light govern what the painter sees at a distance. The upper figure shows towers a, b, c of equal height whose summits appear darker the more degrees of fog lie between them and the eye. The lower figure, a profile head with a sky-arc, is used to explain which parts of the face (a to e) are reached by the sky f k, i k, h k, g k, so that the breast reads as bright as the forehead, nose and chin. The final passage observes that at increasing distance a face loses its finer shadows until only the deepest spots, such as the eye-socket, remain and the whole face reads dark.
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Tower-tops darkened by degrees of fog
The eye seeing towers a b c of equal height sees c, the summit of the nearest tower, at r, a lowness of two degrees of depth in the fog, while it sees the summit of the middle tower b in a single degree of fog. Therefore summit c shows itself darker than the summit of tower b.
Which parts of the face the sky illuminates (a to e)
In a no part of the sky f k illuminates the face; in b the sky i k reaches it, in c the sky h k, in d the sky g [k], and in e the sky f k illuminates it entirely. As a result the breast will be of equal brightness with the forehead, nose and chin.
A face loses its shadow-detail with distance
At various distances the face loses different qualities of shadow, but only the first spots remain, that is the socket of the eye and other similar ones. In the end the face is left dark, because the small lights are consumed against the middle shadows.