... roughly estimating a mile at less than it really is, also in courtesy to them, or at 5000 feet, we have this difference between tints produced by 50,000 feet of air. Then, ten feet of air will produce the 5000th part of this difference. Let the reader take the two extreme tints, and carefully gradate the one into the other. Let him divide this gradated shadow or color into 5000 successive parts; and the difference in depth between one of these parts and the next is the exact amount of aerial perspective between one object, and another, ten feet behind it, ... — The Stones of Venice, Volume III (of 3) • John Ruskin
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