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IX. The Practice of Painting Index
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Gradations of light and shade.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:-- 261 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.
276:261 7: See Pl. XXXI, No. 1; the two upper sketches.