OF CLOUDS, SMOKE AND DUST AND THE FLAMES OF A FURNACE OR OF A BURNING KILN.
The clouds do not show their rounded forms excepting on the sides which face the sun; on the others the roundness is imperceptible because they are in the shade. [Footnote: The text of this chapter is given in facsimile on Pls. XXXVI and XXXVII. The two halves of the leaf form but one in the original. On the margin close to lines 4 and 5 is the note: rossore d’aria inverso l’orizonte—(of the redness of the atmosphere near the horizon). The sketches on the lower portion of the page will be spoken of in No. 668.]
If the sun is in the East and the clouds in the West, the eye placed between the sun and the clouds sees the edges of the rounded forms composing these clouds as dark, and the portions which are surrounded by this dark [edge] are light. And this occurs because the edges of the rounded forms of these clouds are turned towards the upper or lateral sky, which is reflected in them.
Both the cloud and the tree display no roundness at all on their shaded side.
Taken from The Notebooks of Leonardo da Vinci edited by Jean Paul Richter, 1880.