Supposing a b to be the picture and d to be the light, I say that if you place yourself between c and e you will not understand the picture well and particularly if it is done in oils, or still more if it is varnished, because it will be lustrous and somewhat of the nature of a mirror.  And for this reason the nearer you go towards the point c, the less you will see, because the rays of light falling from the window on the picture are reflected to that point.  But if you place yourself between e and d you will get a good view of it, and the more so as you approach the point d, because that spot is least exposed to these reflected rays of light.

III.

THE PRACTICAL METHODS OF LIGHT AND SHADE AND AERIAL PERSPECTIVE.

Gradations of light and shade.

548.

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).

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.