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Paris Manuscript F — 42

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Bibliothèque de l'Institut de France, Paris

Quando il fumo di legne secche si trova in fra l'occhio di chi lo vede e altro loco oscuro, esso pare azzurro e tanto più, quant'è più scuro1.

Adunque l'aria si fa azzurra per le tenebre che essa ha dopo sé. E se tu guardi in verso l'orizzonte del cielo, tu vedrai l'aria non essere azzurra, e questo nasce per la sua grossezza. E così in ogni grado che tu alzi l'occhio sopra esso orizzonte insino al cielo che ti sta di sopra, tu troverrai l'aria farsi più oscura; e questo è che men somma d'aria s'interpone in fra l'occhio tuo e esse tenebre. E se tu2 ti troverrai sopra un alto monte, l'aria si farà tanto più oscura sopra di te, quanto essa è fatta più sottile in fra te e dette tenebre; e così seguiterà in ogni grado d'altezza tanto che alfine resterà tenebrosa.

Quel fumo parrà più azzurro che nascerà di più secche legne, e che sarà più presso alla sua cagione, e che è veduto in più oscuro campo dandovi su il lume del sole.

Why smoke and the air appear blue

When the smoke of dry wood is found between the eye of the one who sees it and some other dark place, it appears blue, and the more so the darker it is.

Therefore the air becomes blue because of the darkness that lies behind it. And if you look toward the horizon of the sky, you will see that the air is not blue, and this arises from its thickness. And so, at every degree by which you raise your eye above that horizon up to the sky that is above you, you will find the air growing darker; and this is because a lesser amount of air is interposed between your eye and those shadows. And if you find yourself upon a high mountain, the air will grow so much darker above you as it has become thinner between you and the said shadows; and so it will continue at every degree of height until at last it will remain wholly dark.

That smoke will appear more blue which is born of drier wood, and which is nearer to its cause, and which is seen against a darker background with the light of the sun falling upon it.

Why smoke and the sky appear blue

The air turns blue against the darkness behind it and grows darker with height and thinness.

Optics & Light Painting & Perspective

A single continuous note in Notebook F explains why smoke and the sky appear blue. Leonardo observes that the smoke of dry wood looks blue when seen against a dark background, and infers that the air itself turns blue because of the darkness lying behind it. He argues that the air near the horizon is not blue because of its thickness, and that it grows darker as the eye is raised toward the zenith or as one climbs a high mountain, where less and thinner air is interposed between the eye and the darkness beyond, until at great height the sky is wholly dark. He adds that smoke looks bluest when it comes from the driest wood, is nearest its source, and is lit by the sun against a dark field.

On this page

Blue arises from darkness seen through thin bright air

Smoke of dry wood, seen between the eye and a dark place, appears blue, and the more so the darker the ground. Leonardo concludes that the air becomes blue because of the darkness lying behind it, lit in front; smoke looks bluest when the wood is driest, nearest its source, and struck by sunlight against a dark background.

The air darkens with height and thinness

Toward the horizon the air is not blue, owing to its thickness; as the eye is raised toward the sky overhead it grows darker, because less air lies between the eye and the darkness beyond. On a high mountain the air above darkens further as it thins, until at last it becomes wholly dark, the basis of aerial perspective.

Reader's Edition — described with AI from the e-Leo transcription.