by which, as
stated in the text, an appearance of retirement or projection may be
obtained, so that they partly supply the place of the stereoscopic
effect, but they do not imitate that effect. The principle of the human
sight is simply this:--by means of our two eyes we literally see
everything from two places at once; and, by calculated combination, in
the brain, of the facts of form so seen, we arrive at conclusions
respecting the distance and shape of the object, which we could not
otherwise have reached. But it is just as vain to hope to paint at once
the two views of the object as seen from these two places, though only
an inch and a half distant from each other, as it would be if they were
a mile and a half distant from each other. With the right eye you see
one view of a given object, relieved against one part of the distance;
with the left eye you see another view of it, relieved against another
part of the distance. You may paint whichever of those views you please;
you cannot paint both. Hold your finger upright, between you and this
page of the book, about six inches from your eyes, and three from the
book; shut the right eye, and hide the words "inches from," in the
second line above this, with your finger; you will then see "six" on one
side of it, and "your," on the other. Now shut the left eye and open the
right without moving your finger, and you will see "inches," but not
"six." You may paint the finger with "inches" beyond it, or with "six"
beyond it, but not with both. And this principle holds for any object
and any distance. You might just as well try to paint St. Paul's at once
from both ends of London Bridge as to realize any stereoscopic effect in
a picture.

NOTE 2, p. 59.--"_Dark lines turned to the light._"

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.

NOTE 3, p. 98.--"_Softness of reflections._"

249. I have not quite insisted enough on the extreme care which is
necessary in giving the tender evanescence of th