world who would not think it necessary
to show pleasure—yes, and feel some indication
of it—over sunset colours, when, by
chance, he treads the fields upon a summer
evening. We all look up at the stars, and
feel that they would seem much less the
confidential friends they really are, if they were
shining down upon us with a rigid light.
There is a beating human pulse which
answers to our hearts in their incessant
twinkling. And then the rainbow! Light
that might pass down to us, and give us
sight, but nothing more, gives sight and
blesses it at once. Its touch converts the
air into a region of delightful visions, ever-
changing, ever new. To reach us it must
penetrate our atmosphere, and it is a fact
that He who made the Universe, so made it
that, in the whole range of Nature there is
not one barren combination. Light must
pass through the air; and, from a knowledge
of the other laws of Nature, it might confidently
be proclaimed, that in addition to
the useful purposes of each, and their most
necessary action on each other, beauty and
pleasure would be generated also by their
union, to delight the creatures of this world.

It is not our design just now to talk about
the nature of the atmosphere; to attempt
any analysis of light, or even to mention its
recondite mysteries. But in a plain way we
propose to look into the reason of those
changes made by light in the appearance of
the sky, those every-day sights with which we
are the most familiar.

Blue sky itself, for example. Why is the
sky blue? To explain that, we must state a
few preliminary facts concerning light, and
beg pardon of any one whose wisdom may be
outraged by the elementary character of our
information. There are some among our
readers, no doubt, who may find it useful.—
In the first place, then, we will begin with
the erection of a pole upon a play-ground, and,
like boys and girls, we will go out to play
about it with an india-rubber ball. The pole
being planted upright, is said to be planted at
right angles to the surface of the ground.
Now, if we climb the pole, and throw our
ball down in the same line with it, it will run
down the pole and strike the ground, and then
jump back again by the same road into our
fingers. The bouncing back is called in
scientific phrase, Reflection; and so we may
declare about our ball, that if it strike a plane
surface at right angles, it is reflected immediately
back upon the line it went by, or, as
scientific people say, "the line of incidence."
Now, let us walk off, and mount a wall at a
short distance from the pole. We throw our
ball so that it strikes the ground quite close to
the spot at which the pole is planted in the
earth, and we observe that the said ball no
longer returns into our hand, but flies up,
without deviating to the right or left (in the
same plane, says Science) beyond the pole,
with exactly the same inclination towards the
pole on one side, and the surface of the ground
on the other, as we gave it when we sent it
down. So if there were a wall on the other
side of our pole, exactly as distant and as high
as our own, and somebody should sit thereon
directly opposite to us, the ball would shoot
down from our fingers to the root of the pole,
and then up from the pole into his hand.
Spread a string on each side along the course
the ball has taken, from wall to pole, and
from pole to wall. The string on each side
will make with the pole an equal angle: the
angle to the pole, by which the ball went, is
called, we said, the angle of incidence; the
angle from the pole by which it bounced off,
is called the angle of reflection. Now, it is
true not only of balls, but of all things that
are reflected; of light, for example, reflected
from a looking-glass, or a sheet of water, that
"the angle of reflection is equal to the angle
of incidence."

The light that shines back to us from a
sheet of water, has not penetrated through its
substance, certainly. But now, let us be
Tritons, or sea nymphs, and let us live in a
cool crystal grot under the waves. We don't
live in the dark, unless we be unmitigated
deep-sea Tritons. The deeper we go, the
darker we find it. Why? Now, let us be
absurd, and suppose that it is possible for
light to be measured by the bushel. Ten
bushels of light are poured down from the
sun upon a certain bit of water; six of these,
we will say, reflected from its surface, cause
the glittering appearance, which is nothing to
us Tritons down below. But light can pass
through water; that is to say, water is a
transparent substance; so the other four
bushels soak down to illuminate the fishes.
But this light, so soaking down, is by the
water (and would be by any other transparent
substance) absorbed, altered, partly
converted into heat—when we understand
exactly what Mr. Grove calls the Correlation
of Physical Forces, we shall understand the
why and how—we only know just now the
fact, that all transparent bodies do absorb
and use up light; so that the quantity of
light which entered at the surface of our
water suffers robbery, becoming less and
less as it sinks lower down towards our
coral caves.

Furthermore, beside reflection and absorption,
there is one more thing that light suffers;
and that we must understand before we can
know properly why skies are blue, and stars
are twinkling. That one thing more is called
Refraction. A horse trots fairly over the
stones, but slips the moment stones end, and
he comes upon wood pavement. A ray of
light travels straight as a dancing-master's
back, so long as it is in air, or water, or glass,
or any other "medium," as the books say, of
a certain unvarying thinness or thickness,
fineness or coarseness, or according to the
school-word "density." But if a ray that has
been travelling through warm and light air,