OF JUDGING YOUR OWN PICTURES.

We know very well that errors are better recognised in the works of
others than in our own; and that often, while reproving little
faults in others, you may ignore great ones in yourself. To avoid
such ignorance, in the first place make yourself a master of
perspective, then acquire perfect knowledge of the proportions of
men and other animals, and also, study good architecture, that is so
far as concerns the forms of buildings and other objects which are
on the face of the earth; these forms are infinite, and the better
you know them the more admirable will your work be. And in cases
where you lack experience do not shrink from drawing them from
nature. But, to carry out my promise above [in the title]--I say
that when you paint you should have a flat mirror and often look at
your work as reflected in it, when you will see it reversed, and it
will appear to you like some other painter's work, so you will be
better able to judge of its faults than in any other way. Again, it
is well that you should often leave off work and take a little
relaxation, because, when you come back to it you are a better
judge; for sitting too close at work may greatly deceive you. Again,
it is good to retire to a distance because the work looks smaller
and your eye takes in more of it at a glance and sees more easily
the discords or disproportion in the limbs and colours of the
objects.

On the management of works (531. 532).