OF PAINTING: OF THE DARKNESS OF THE SHADOWS, OR I MAY SAY, THE
BRIGHTNESS OF THE LIGHTS.

Although practical painters attribute to all shaded objects--trees,
fields, hair, beards and skin--four degrees of darkness in each
colour they use: that is to say first a dark foundation, secondly a
spot of colour somewhat resembling the form of the details, thirdly
a somewhat brighter and more defined portion, fourthly the lights
which are more conspicuous than other parts of the figure; still to
me it appears that these gradations are infinite upon a continuous
surface which is in itself infinitely divisible, and I prove it
thus:--[Footnote 7: See Pl. XXXI, No. 1; the two upper sketches.]
Let _a g_ be a continuous surface and let _d_ be the light which
illuminates it; I say--by the 4th [proposition] which says that that
side of an illuminated body is most highly lighted which is nearest
to the source of light--that therefore _g_ must be darker than _c_
in proportion as the line _d g_ is longer than the line _d c_, and
consequently that these gradations of light--or rather of shadow,
are not 4 only, but may be conceived of as infinite, because _c d_
is a continuous surface and every continuous surface is infinitely
divisible; hence the varieties in the length of lines extending
between the light and the illuminated object are infinite, and the
proportion of the light will be the same as that of the length of
the lines between them; extending from the centre of the luminous
body to the surface of the illuminated object.

On the choice of light for a picture (549-554).