Distance and color of peaks are not in the same ratio
With peaks o, p, q equally spaced (a o = 2, a p = 4, a q = 6), the air they are seen through is not in equal proportion
This page (chapter 795) argues that when the eye is higher than several equally spaced peaks, the diminution of their color does not follow the same proportion as their distances, because their images pass through different thicknesses of air. Leonardo lets o, p, q be three peaks of one color equally distant from one another, and a the eye above them. With the distances a o, a p, a q as two, four, and six, the air over the nearer peak is not sub-double but sub-triple relative to the next, so the color falls out of step with the distance. A small diagram inset in the text carries the labels o, p, q, s, m, n and the eye a.
On this page
The eye sees peaks b, c, d merged in the boundary of the first peak b
The peak b coincides with the two other peaks c and d, so the eye a sees the three peaks b, c, d in one same boundary of mountain b. These peaks have their distances and colors in the same proportion, but neither distance nor color can be distinguished.
Distances a o, a p, a q as 2, 4, 6 against the thickness of air
The distances a o, a p, a q stand as two, four, and six, in the proportion of equality of intervals. Yet the air n o is not sub-double to the air m p but sub-triple, and the space a o is sub-double to the space a p and sub-quadruple to the space a q, so the color-diminution does not match the distance-ratio.
Equally spaced peaks are seen through unequal thicknesses of air
When the eye sees the peaks of mountains at equal distances and heights below it, it will not see their colors diminished in the same proportion as those distances, because the images pass to the eye through different thicknesses of air.
Inset diagram of three peaks with the eye above (labels o, p, q, s, m, n, a)
A small ruled drawing set into the text shows a low range of peaks with an eye above and sight-lines drawn to each summit; letters o, p, q mark the peaks, s, m, n mark points along the air, and a marks the eye. It illustrates the mismatch between distance and color-diminution.