Why a Dark Cloud Looks Higher; Painted vs. Real Distance
The visual pyramid cut on the wall at m a and n m, and the paradox of two things seen at one angle
The page explains why a lower, darker cloud can seem higher than a higher, lighter one: on the wall the visual pyramid of the low cloud is cut at m a and that of the higher cloud at n m, below a n, an effect seen when sun rays strike clouds in the east or west. A new precept then poses the problem that a painted thing reaching the eye at the same angle as a truly remote object does not look as far away as the natural distance, introducing the demonstration in which one thing is painted on the wall b c to appear a mile off with another set beside it. Two ink diagrams accompany the text: an upper cloud-and-sun scheme lettered a, n, m with SOLE, and a lower cone-of-vision scheme with an eye and points A, B, C.
On this page
A low dark cloud appearing higher than a light one
The first cloud, though lower than the second, appears higher, because on the wall the pyramid of the low cloud is cut at m a and that of the higher one at n m, below a n. This happens when the sun's rays, in the east or west, make a dark cloud seem higher than a light one.
Painted distance against natural distance
Although a painted thing reaches the eye by the same width of angle as one that is truly more remote, it does not appear as distant as the object at its natural remove. To test this, a thing is painted on the wall b c to seem a mile away, and beside it is set another that has the true distance.
Diagrams: cloud pyramid and the eye's cone
The upper drawing shows clouds and the sun (SOLE) with a vertical wall marked at points a, n and m, illustrating the cutting of the pyramids. The lower drawing shows a circle and an eye at the right with points A, B and C, illustrating how bodies at one visual angle compare in apparent distance.