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.
116.
Let _a_ be the light, _b_ the eye, _c_ the object seen by the eye
and in the light. These show, first, the eye between the light and
the body; the 2nd, the light between the eye and the body; the 3rd
the body between the eye and the light, _a_ is the eye, _b_ the
illuminated object, _c_ the light.
117.