248. It ought to have been farther observed, that the inclosure of the
light by future shadow is by no means the only reason for the dark lines
which great masters often thus introduce. It constantly happens that a
local color will show its own darkness most on the light side, by
projecting into and against masses of light in that direction; and then
the painter will indicate this future force of the mass by his dark
touch. Both the monk's head in Fig. 11 and dog in Fig. 20 are dark
towards the light for this reason.