OF PAINTING.

As regards all visible objects 3 things must be considered. These
are the position of the eye which sees: that of the object seen
[with regard] to the light, and the position of the light which
illuminates the object, _b_ is the eye, _a_ the object seen, _c_ the
light, _a_ is the eye, _b_ the illuminating body, _c_ is the
illuminated object.