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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.]
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