When you want to see if your picture corresponds throughout with the
objects you have drawn from nature, take a mirror and look in that
at the reflection of the real things, and compare the reflected
image with your picture, and consider whether the subject of the two
images duly corresponds in both, particularly studying the mirror.
You should take the mirror for your guide--that is to say a flat
mirror--because on its surface the objects appear in many respects
as in a painting. Thus you see, in a painting done on a flat
surface, objects which appear in relief, and in the mirror--also a
flat surface--they look the same. The picture has one plane surface
and the same with the mirror. The picture is intangible, in so far
as that which appears round and prominent cannot be grasped in the
hands; and it is the same with the mirror. And since you can see
that the mirror, by means of outlines, shadows and lights, makes
objects appear in relief, you, who have in your colours far stronger
lights and shades than those in the mirror, can certainly, if you
compose your picture well, make that also look like a natural scene
reflected in a large mirror.
[Footnote: I understand the concluding lines of this passage as
follows: If you draw the upper half a figure on a large sheet of
paper laid out on the floor of a room (_sala be piana_) to the same
scale (_con le sue vere grosseze_) as the lower half, already drawn
upon the wall (lines 10, 11)you must then reduce them on a '_pariete
di rilievo_,' a curved vertical plane which serves as a model to
reproduce the form of the vault.]
530.