How Surfaces Take False Colours: Yellow and Blue Make Green
A blue body under a yellow object looks green; a shadow lit by two lights takes a false hue
After finishing the window example, this folio opens chapter [701], on how opaque surfaces take on colours that are not in the objects colouring them. Leonardo lets the opaque body c d be blue and its object a b be yellow, and shows that the surface d n c appears green, because when different colours mix they become a third colour partaking of both, so yellow with blue makes green as the speculative painter well understands. Chapter [702], on the false colour of shadows, sets a white opaque body n d e lit by the air a b and by fire c q, then interposes an object o p so that its shadow is cut on the surface at d n. A pen diagram with a circle and lettered points sits in the text column.
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[701] Yellow object on a blue body reads green
Let c d be the opaque body and a b its object, the object yellow and the body blue; the surface d n c, though blue in itself, will appear green, and the reverse mixture gives the same. This happens because different colours, when mixed, are transmuted into a third colour partaking of both, so yellow mixed with blue makes green.
Surfaces take colours not present in their objects
Many times the surfaces of opaque bodies, in being tinged by the colours of their objects, take on colours that are not in those objects at all. Leonardo says this compounded green is manifestly understood by the speculative painter, framing colour mixing as knowledge the painter must possess.
[702] The false colour of a shadow lit by two lights
When an opaque body casts its shadow on another opaque body lit by two different lights, the shadow does not appear to belong to that body but to something else. Here the white body n d e is lit by the air a b and by the fire c q, and the interposed object o p cuts its shadow on the surface at d n, where the redness no longer reaches.
Diagram of the two-light shadow experiment
A small pen diagram inset in the column shows a circle with tangent lines and lettered points, laying out the opaque body, the two light sources and the interposing object. It is drawn, not transcribed, and keys the points a, b, c, d, e, n, o, p, q of the proof.