out curvature, and
every curved surface must be gradated by the nature of light, which is
most intense when it impinges at the highest angle, and for the
gradation of the few plane surfaces that exist, means are provided in
local color, aerial perspective, reflected lights, etc., from which it
is but barely conceivable that they should ever escape. Hence for
instances of the complete absence of gradation we must look to man's
work, or to his disease and decrepitude. Compare the gradated colors of
the rainbow with the stripes of a target, and the gradual concentration
of the youthful blood in the cheek with an abrupt patch of rouge, or
with the sharply drawn veining of old age.

Sec. 17. How found in Nature.

Gradation is so inseparable a quality of all natural shade and color
that the eye refuses in art to understand anything as either, which
appears without it, while on the other hand nearly all the gradations of
nature are so subtile and between degrees of tint so slightly separated,
that no human hand can in any wise equal, or do anything more than
suggest the idea of them. In proportion to the space over which
gradation extends, and to its invisible subtilty, is its grandeur, and
in proportion to its narrow limits and violent degrees, its vulgarity.
In Correggio, it is morbid and vulgar in spite of its refinement of
execution, because the eye is drawn to it, and it is made the most
observable and characteristic part of the picture; whereas natural
gradation is forever escaping observation to that degree that the
greater part of artists in working from nature see it not, (except in
certain of its marked developments,) but either lay down such
continuous lines and colors, as are both disagreeable and impossible,
or, receiving the necessity of gradation as a principle instead of a
fact, use it in violently exaggerated measure, and so lose both the
dignity of their own work, and by the constant dwelling of their eyes
upon exaggerations, their sensibility to that of the natural forms. So
that we find the majority of painters divided between the two evil
extremes of insufficiency and affectation, and only a few of the
greatest men capable of making gradation constant and yet extended over
enormous spaces and within degrees of narrow difference, as in the body
of a high light.

Sec. 18. How necessary in Art.

From the necessity of gradation results what is commonly given as a rule
of art, though its authority as a