227. Closely connected with the law of contrast is a law which enforces
the unity of opposite things, by giving to each a portion of the
character of the other. If, for instance, you divide a shield into two
masses of color, all the way down--suppose blue and white, and put a
bar, or figure of an animal, partly on one division, partly on the
other, you will find it pleasant to the eye if you make the part of the
animal blue which comes upon the white half, and white which comes upon
the blue half. This is done in heraldry, partly for the sake of perfect
intelligibility, but yet more for the sake of delight in interchange of
color, since, in all ornamentation whatever, the practice is continual,
in the ages of good design.
228. Sometimes this alternation is merely a reversal of contrasts; as
that, after red has been for some time on one side, and blue on the
other, red shall pass to blue's side and blue to red's. This kind of
alternation takes place simply in four-quartered shields; in more subtle
pieces of treatment, a little bit only of each color is carried into the
other, and they are as it were dovetailed together. One of the most
curious facts which will impress itself upon you, when you have drawn
some time carefully from Nature in light and shade, is the appearance of
intentional artifice with which contrasts of this alternate kind are
produced by her; the artistry with which she will darken a tree trunk as
long as it comes against light sky, and throw sunlight on it precisely
at the spot where it comes against a dark hill, and similarly treat all
her masses of shade and color, is so great, that if you only follow her
closely, every one who looks at your drawing with attention will think
that you have been inventing the most artificially and unnaturally
delightful interchanges of shadow that could possibly be devised by
human wit.
229. You will find this law of interchange insisted upon at length by
Prout in his Lessons on Light and Shade: it seems of all his principles
of composition to be the one he is most conscious of; many others he
obeys by instinct, but this he formally accepts and forcibly declares.
The typical purpose of the law of interchange is, of course, to teach us
how opposite natures may be helped and strengthened by receiving each,
as far as they can, some impress or reflection, or imparted power, from
the other.