...from nature. But to return to the promise made above, I say that in your painting you should keep a flat mirror and often look within it at your work, which will there be seen reversed, and will seem to you by the hand of another master, and you will judge your errors better there than otherwise. It is also good often to rise up and take a little other diversion, because on returning to the work you improve your judgment; for staying fixed upon the work makes you strongly deceived. It is good also to move away, because the work seems smaller, and more is comprehended at a single glance, and the discordant and disproportionate limbs and the colours of things are better recognised than from close up.

[408] How the mirror is the master of painters. When you wish to see whether your painting as a whole has conformity with the thing portrayed from nature, take a mirror, and cause the living thing to be mirrored within it, and compare the mirrored thing with your painting, and consider well whether the subject of the one and the other likeness has conformity together. And above all the mirror should be taken as your master, that is, the flat mirror, because on its surface things have likeness with painting in many respects; that is, you see the painting made upon a plane show things that appear in relief, and the mirror upon a plane does the same; the painting is a single surface, and the mirror is that same thing; the painting is impalpable, in that what appears round and detached cannot be encircled with the hands, and the mirror does the like. The mirror and the painting show the likenesses of things surrounded by shadow and light, and the one and the other appear far beyond their surface.