other. If, for instance, you divide a shield into two masses of
colour, 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
colour, since, in all ornamentation whatever, the practice is
continual, in the ages of good design.

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 colour 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 colour, 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 artifically and unnaturally
delightful interchanges of shadow that could possibly be devised by
human wit.

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 imparted power, from the other.

8. THE LAW OF CONSISTENCY.

It is to be remembered, in the next place, that while contrast exhibits
the _characters_ of things, it very often neutralises or paralyses their
_power_. A number of white things may be shown to be clearly white by
op