THE PROOF AND REASON WHY AMONG THE ILLUMINATED PARTS CERTAIN
PORTIONS ARE IN HIGHER LIGHT THAN OTHERS.

Since it is proved that every definite light is, or seems to be,
derived from one single point the side illuminated by it will have
its highest light on the portion where the line of radiance falls
perpendicularly; as is shown above in the lines _a g_, and also in
_a h_ and in _l a_; and that portion of the illuminated side will be
least luminous, where the line of incidence strikes it between two
more dissimilar angles, as is seen at _b c d_. And by this means you
may also know which parts are deprived of light as is seen at _m k_.

Where the angles made by the lines of incidence are most equal there
will be the highest light, and where they are most unequal it will
be darkest.

I will make further mention of the reason of reflections.

[Footnote: See Pl. XXXII. The text, here given complete, is on the
right hand side. The small circles above the beginning of lines 5
and 11 as well as the circle above the text on Pl. XXXI, are in a
paler ink and evidently added by a later hand in order to
distinguish the text as belonging to the _Libro di Pittura_ (see
Prolegomena. No. 12, p. 3). The text on the left hand side of this
page is given as Nos. 577 and 137.]