Precepts of painting: confused edges at a distance
Why distant things blur into blue, and why lower things look more confused than high ones
Two lettered precepts of painting treat the loss of definition over distance. Precept [473] warns that making distant things crisp and known makes them read as near, so the painter should give things only the degree of clarity their distance shows, and imitate confused edges where the object has them. It gives two causes of this confusion: the small visual angle that reduces a body to the illegibility of the smallest near things (like the nails on an ant's fingers), and the air, thick and white, that veils shadows into a blue between black and white. Precept [474] adds that sunlit distant things read more certainly than others lost in mist, and that lower things appear more confused because the air grows thicker toward the ground.
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Give distant things only the clarity their distance shows
If you make things known and distinct in long distances, they will show themselves not distant but near. So in your imitation give things that degree of cognition which the distances show, and if the object has confused and doubtful edges, do the like in your image.
Two causes of the confused distant edge
Distant things blur for two causes. First, they come to the eye by so small an angle that they act like the smallest near things, whose figure the eye cannot grasp, as the nails on the fingers of ants. Second, so much air interposes that, thick and white, it tinges and veils the shadows into a color between black and white, which is blue.
Sunlit things read clearer; low air is thicker
Although long distances lose the knowledge of many things, those lit by the sun render themselves more certain while the rest appear wrapped in confused mists. Because at every degree of lowness the air gains thickness, lower things show themselves more confused, and higher things conversely.