WHY A PAINTING CAN NEVER APPEAR DETACHED AS NATURAL OBJECTS DO.

Painters often fall into despair of imitating nature when they see
their pictures fail in that relief and vividness which objects have
that are seen in a mirror; while they allege that they have colours
which for brightness or depth far exceed the strength of light and
shade in the reflections in the mirror, thus displaying their own
ignorance rather than the real cause, because they do not know it.
It is impossible that painted objects should appear in such relief
as to resemble those reflected in the mirror, although both are seen
on a flat surface, unless they are seen with only one eye; and the
reason is that two eyes see one object behind another as _a_ and _b_
see _m_ and _n_. _m_ cannot exactly occupy [the space of] _n_
because the base of the visual lines is so broad that the second
body is seen beyond the first. But if you close one eye, as at _s_
the body _f_ will conceal _r_, because the line of sight proceeds
from a single point and makes its base in the first body, whence the
second, of the same size, can never be seen.

[Footnote: This passage contains the solution of the problem
proposed in No. 29, lines 10-14. Leonardo was evidently familiar
with the law of optics on which the construction of the stereoscope
depends. Compare E. VON BRUCKE, _Bruchstucke aus der Theorie der
bildenden Kunste_, pg. 69: "_Schon Leonardo da Vinci wusste, dass
ein noch so gut gemaltes Bild nie den vollen Eindruck der
Korperlichkeit geben kann, wie ihn die Natur selbst giebt. Er
erklart dies auch in Kap. LIII und Kap. CCCXLI_ (ed. DU FRESNE)
_des_ 'Trattato' _in sachgemasser Weise aus dem Sehen mit beiden
Augen_."

Chap. 53 of DU FRESNE'S edition corresponds to No. 534 of this
work.]