Second principle of chiaroscuro

The  second  point  to  which  I  wish  at  present  to  direct attention has reference to the arrangement of light and shade. It is the constant habit of nature to use both  her  highest  lights  and  deepest  shadows  in exceedingly  small  quantity;  always  in  points, never  in  masses.  She  will  give  a  large  mass  of  tender  light  in  sky  or  water,  impressive  by  its quantity,  and  a  large  mass  of  tender  shadow relieved against it, in foliage, or hill, or building; but the light is always subdued if it be extensive, the shadow always feeble if it be  broad.  She  will  then  fill  up  all  the  rest  of  her  picture  with middle tints and pale greys of some sort or another, and on this quiet  and  harmonious  whole  she  will  touch  her  high  lights  in spots: the foam of an isolated wave, the sail of a solitary vessel, the  flash  of  the  sun  from  a  wet  roof,  the  gleam  of  a  single white-washed cottage, or some such sources of local brilliancy, she will use so vividly and delicately as to throw everything else into definite shade by comparison. And then taking  up  the  gloom,  she  will  use the  black  hollows  of  some overhanging bank, or the black dress of some shaded figure, or the depth of some sunless chink of wall or window, so sharply as to throw everything else into definite light by comparison; thus reducing the whole mass of her picture to a delicate middle tint, approaching, of course, here to light, and there to gloom; but yet sharply separated from the utmost degrees either of the one or the other.

....

And it is most singular that this separation, which is the great source of brilliancy in nature, should not only be unobserved, but absolutely forbidden, by our great writers  on  art,  who  are  always  talking  about connecting    the    light    with    the    shade    by imperceptible  gradations.  Now  so  surely  as  this  is  done,  all sunshine is lost, for imperceptible gradation from light to dark is the characteristic of objects seen out of sunshine, in what is, in landscape,  shadow.  Nature's  principle  of  getting  light  is  the direct reverse. She will cover her whole landscape with middle tint, in which she will have as many gradations as you please, and a great many more than you can paint; but on this middle tint she touches her extreme lights, and extreme darks, isolated and sharp, so that the eye goes to them directly, and feels them to be keynotes  of  the  whole  composition. And  although  the  dark touches are less attractive than the light ones,  it  is  not  because  they  are  less  distinct,  but  because  they exhibit  nothing;  while  the  bright  touches  are  in  parts  where everything is seen, and  where in consequence the eye  goes to rest.