If you want to represent a figure on a wall, the wall being
foreshortened, while the figure is to appear in its proper form, and
as standing free from the wall, you must proceed thus: have a thin
plate of iron and make a small hole in the centre; this hole must be
round. Set a light close to it in such a position as that it shines
through the central hole, then place any object or figure you please
so close to the wall that it touches it and draw the outline of the
shadow on the wall; then fill in the shade and add the lights; place
the person who is to see it so that he looks through that same hole
where at first the light was; and you will never be able to persuade
yourself that the image is not detached from the wall.

[Footnote: _uno piccolo spiracelo nel mezzo_. M. RAVAISSON, in his
edition of MS. A (Paris), p. 52, reads _nel muro_--evidently a
mistake for _nel mezzo_ which is quite plainly written; and he
translates it _"fait lui une petite ouverture dans le mur,"_ adding
in a note: _"les mots 'dans le mur' paraissent etre de trop.
Leonardo a du les ecrire par distraction"_ But _'nel mezzo'_ is
clearly legible even on the photograph facsimile given by Ravaisson
himself, and the objection he raises disappears at once. It is not
always wise or safe to try to prove our author's absence of mind or
inadvertence by apparent difficulties in the sense or connection of
the text.]