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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).
391
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