drawing_ of objects, that is to say, the degree in which their
details and parts are distinct or confused, is an unfailing and certain
criterion of their distance; and if this be rightly rendered in a
painting, we shall have genuine truth of space, in spite of many errors
in aerial tone; while, if this be neglected, all space will be
destroyed, whatever dexterity of tint may be employed to conceal the
defective drawing.

Sec. 2. It is impossible to see objects at unequal distances distinctly at
one moment.

First, then, it is to be noticed, that the eye, like any other lens,
must have its focus altered, in order to convey a distinct image of
objects at different distances; so that it is totally impossible to see
distinctly, at the same moment, two objects, one of which is much
farther off than another. Of this, any one may convince himself in an
instant. Look at the bars of your window-frame, so as to get a clear
image of their lines and form, and you cannot, while your eye is fixed
on them, perceive anything but the most indistinct and shadowy images of
whatever objects may be visible beyond. But fix your eyes on those
objects, so as to see them clearly, and though they are just beyond and
apparently beside the window-frame, that frame will only be felt or seen
as a vague, flitting, obscure interruption to whatever is perceived
beyond it. A little attention directed to this fact will convince every
one of its universality, and prove beyond dispute that objects at
unequal distances cannot be seen together, not from the intervention of
air or mist, but from the impossibility of the rays proceeding from
both, converging to the same focus, so that the whole impression, either
of one or the other, must necessarily be confused, indistinct, and
inadequate.

Sec. 3. Especially such as are both comparatively near.

But, be it observed (and I have only to request that whatever I say may
be tested by immediate experiment,) the difference of focus necessary is
greatest within the first five hundred yards, and therefore, though it
is totally impossible to see an object ten yards from the eye, and one a
quarter of a mile beyond it, at the same moment, it is perfectly
possible to see one a quarter of a mile off, and one five miles beyond
it, at the same moment. The consequence of this is, practically, that in
a real landscape, we can see the whole of what would be called the
middle distance and distance together, with