Labels:text | cloud | screenshot | sky OCR: Leonardo's experiment is successful if the black is a bluish black, as were many of the black pigments available in his day. An "Overlay' of this type occurs in nature, with the whiteness of illuminated air functioning as a white glaze over the darkness of the outer sphere. Leonardo describes how to test this effect by observing fine sprays of water and smoke in front of a black velvet cloth.
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