Of the houses of a town, in which the divisions between the houses may be distinguished by the light which fall on the mist at the bottom. If the eye is above the houses the light seen in the space that is between one house and the next sinks by degrees into thicker mist; and yet, being less transparent, it appears whiter; and if the houses are some higher than the others, since the true [colour] is always more discernible through the thinner atmosphere, the houses will look darker in proportion as they are higher up. Let n o p q represent the various density of the atmosphere thick with moisture, a being the eye, the house b c will look lightest at the bottom, because it is in a thicker atmosphere; the lines c d f will appear equally light, for although f is more distant than c, it is raised into a thinner atmosphere, if the houses b e are of the same height, because they cross a brightness which is varied by mist, but this is only because the line of the eye which starts from above ends by piercing a lower and denser atmosphere at d than at b. Thus the line a f is lower at f than at c; and the house f will be seen darker at e from the line e k as far as m, than the tops of the houses standing in front of it.
Taken from The Notebooks of Leonardo da Vinci edited by Jean Paul Richter, 1880.