l _cd_.
First, therefore, put your paper upright at _e_, and then bring it
gradually to the window, in the successive positions _f_, _g_, and
(opening the window) finally at _p_. You will notice that as you come
nearer the window the light gradually _increases_ on the paper; so that
in the position at _p_ it is far better lighted than it was at _e_. If,
however, the sun actually falls upon it at _p_, the experiment is
unfair, for the picture is not meant to be seen in sunshine, and your
object is to compare pure white paper, as ordinarily used, _with_
sunshine. So either take a time when the sun does not shine at all, or
does not shine in the window where the experiment is to be tried; or
else keep the paper so far within the window that the sun may not touch
it. Then the experiment is perfectly fair, and you will find that you
have the paper at _p_ in full, serene, pictorial light, of the best
kind, and highest attainable power.

[Illustration: FIG. 2.]

Sec. 2. Now, leaning a little over the window sill, bring the edge of the
paper at _p_ against the sky, rather low down on the horizon (I suppose
you choose a fine day for the experiment, that the sun is high, and the
sky clear blue, down to the horizon). The moment you bring your white
paper against the sky you will be startled to find this bright white
paper suddenly appear in shade. You will draw it back, thinking you have
changed its position. But no; the paper is not in shade. It is as bright
as ever it was; brighter than under ordinary circumstances it ever can
be. But, behold, the blue sky of the horizon is far brighter. The one is
indeed blue, and the other white, but the _white_ is _darkest_,[16] and
by a great deal. And you will, though perhaps not for the first time in
your life, perceive that though black is not easily proved to be white,
white, may, under certain circumstances, be very nearly proved black, or
at all events brown.

Sec. 3. When this fact is first show to them, the general feeling with most
people is, that, by being brought against the sky, the white paper is
somehow or other brought "into shade." But this is not so; the paper
remains exactly as it was; it is only compared with an actually brighter
hue, and looks darker by comparison. The circumstances are precisely
like those which affect our sensations of heat and cold. If, when by
chance we have one hand warm, and another cold, we feel, with each hand,
water warmed to an intermediate d