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.]
Taken from The Notebooks of Leonardo da Vinci edited by Jean Paul Richter, 1880.