On Painting: Eye, Object and Light
The three positions to weigh for every visible body, with illustrative diagrams
Headed 'On painting,' this folio sets out a rule for representing anything that is seen: three positions must be considered together, that of the observing eye, that of the object observed, and that of the light that illuminates the body. Two accompanying diagrams apply the rule, one showing a body a lit by c and viewed obliquely by the eye b, the other placing the eye at a with a small illuminant b lighting a larger body c. The lettering a, b, c is reused with different meanings in each figure.
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On painting: eye, object and light
The folio opens with the heading 'On painting.' For every object that is seen, three positions must be considered together: the eye that observes, the object that is observed, and the light that illuminates the body.
Object a seen obliquely, lit by c (eye b)
A first diagram shows a body a lit by the luminous body c, with the eye b viewing it from an oblique angle so that it takes in part of the light and part of the shadow.
Eye a, small illuminant b, illuminated body c
A second diagram places the eye at a, the illuminating body at b (smaller than the body it lights), and the illuminated body at c, with the illuminant falling within the visual lines running from eye to object.