A Treatise on Painting, by Leonardo da Vinci
Senex and Taylor, London
That part of the Air near our Earth, being grosser than that at a greater Distance, of consequence it must receive and reflect a greater share of Light: This you may observe by looking towards the West, when the Sun is rising: for you will see a considerable Brightness in that quarter, before any Appearance of Light be discoverable over your Head. In painting a Landskip, therefore, where the Sight is supposed to be bounded with a large Plain, the Heavens must be represented brighter, in proportion, as they appear near the Earth; the lower Part, at the same time, being seen whiter and more lucid than the higher, in the same proportion; because the Rays of Light reflected from the former, pass through a larger Track of Gross, white Air than those from the latter, and of Consequence must be more tinged, with the whiteness thereof. But in looking towards the East, when the Sun rises, the Air appears more obscure, in proportion as it is lower; the Sunbeams being scarce able to make their way thro' the Gross, Vapoury Air, of the lower Regions.