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). 549. HOW THE PAINTER MUST PLACE HIMSELF WITH REFERENCE TO THE LIGHT, TO GIVE THE EFFECT OF RELIEF. Let _a b_ be the window, _m_ the point of light. I say that on whichever side the painter places himself he will be well placed if only his eye is between the shaded and the illuminated portions of the object he is drawing; and this place you will find by putting yourself between the point _m_ and the division between the shadow and the light on the object to be drawn. 550. THAT SHADOWS CAST BY A PARTICULAR LIGHT SHOULD BE AVOIDED, BECAUSE THEY ARE EQUALLY STRONG AT THE ENDS AND AT THE BEGINNING. The shadows cast by the sun or any other particular light have not a pleasing effect on the body to which they belong, because the parts remain confuse, being divided by distinct outlines of light and shade. And the shadows are of equal strength at the end and at the beginning. 551. HOW LIGHT SHOULD BE THROWN UPON FIGURES.