suddenly plunges into air cold and heavy, it
is put out of the way by such a circumstance,
and in the moment of making such a change,
it alters its direction. Still more, a ray of
light that has been travelling in a straight
line through air, is put out of its course on
entering the denser medium of water; it is
dislocated, refracted very much, alters its
course, and then continues in a straight line on
the new course, so long as the new medium
continues. In the same way, a ray of light
which travels through a medium that becomes
denser and denser very gradually, would be
perpetually swerving from its straight path,
and would travel on a curve. Our atmosphere
is heaviest upon the surface of the earth, and
becomes lighter and thinner as we rise; the
ray, therefore, from a star comes to us after
travelling in such a curve. But we see all
objects in the direction of a perfectly straight
line continued in the direction which the rays
sent from them took at the moment of falling
upon our sense of sight. Therefore we see
all stars in a part of the heavens where they
really are not; we see the sun before it really
rises. Light entering a denser medium is
refracted from, entering a lighter medium is
refracted towards, a line drawn at right angles
to its surface. Light entering a new medium
at right angles—that is to say, not aslant—
continues its course unaltered.

There is but one more fact necessary to fill
up the small measure of preliminary knowledge
necessary for a general understanding
of the phenomena produced by the mixing of
light with air. Light in its perfect state is
white, but the white light is a compound of
other rays in due proportion, each ray being
different in colour and different in quality.
So it takes place, because their qualities are
different, that grass reflects the green ray and
absorbs the rest, and therefore grass is green;
while orange-peel reflects another ray, and
swallows up the green and all the rest. These
colours being in the light, not in the substance
coloured; in a dark room it is not merely a
fact that we cannot see red curtains and
pictures; but the curtains really are not red,
the paintings have no colour in them, till the
morning come, and artfully constructed surfaces
once more in a fixed manner decompose
the light. Beside the colour of these rays,
from which light is compounded, there are
combined with them other subtle principles
which act mysteriously upon matter. Upon
the hard surface of a pebble there are changes
that take place whenever a cloud floats before
the sun. Never mind that now. The coloured
rays of which pure white light is compounded
are usually said to be seven—Violet, Indigo,
Blue, Green, Yellow, Orange, Red; and they
may be technically remembered in their proper
order by combining their initials into the
barbarous word Vibgyor. These are called
prismatic colours, because they were first
separated by the passing of a ray of pure
light through a prism. In that passage light
is much refracted, and it happens that the
contained rays all disagree with one another
as to the extent to which they suffer themselves
to be put out by a change of medium.
Violet refracts most, and Red least; the others
stand between in the order in which they
have just been named, the order in which
you see them in the rainbow. So the rays
after refraction come out in a state of
dissension; all the rays—made refractory—having
agreed to separate, because they are not of
one mind, but of seven minds, about the
degree to which they should be put out by
the trouble they have gone through.

Now we have settled our preliminaries, we
have got our principles; the next thing is to
put them into practice. Let us first note
what has been said of the absorption of light
by transparent bodies. The air is one of the
most transparent bodies known. On a clear
day—when vapour (that is not air) does not
mingle with our atmosphere—mechanical obstacles
and the earth's figure form the only
limits to our vision. You may see Cologne
Cathedral from a mountain distant nearly
sixty miles. Nevertheless, if the atmosphere
had no absorbing power, only direct rays
of the sun, or rays reflected from the substances
about us, would be visible; the sky
would be black, not blue; and sunset would
abruptly pitch us into perfect night. The air,
however, absorbs light, which becomes intermixed
with its whole substance. Hold up
your head, open your eyes widely, and stare at
the noon-day sun. You will soon shut your
eyes and turn your head away; look at him
in the evening or in the morning, and he will
not blind you. Why? Remembering the
Earth to be a globe surrounded by an
atmosphere, you will perceive that the sun's rays
at noonday have to penetrate the simple thickness
of the atmosphere, measured in a straight
line upwards from the earth; but in the
evening or morning its beams fall aslant, and
have to slip through a great deal of air before
they reach us; suffering, therefore, a great
deal of robbery; that is to say, having much
light absorbed.

Now, why is the sky blue? Not only does the
air absorb light; it reflects it also. The particles
of air reflect, however, most especially
the blue ray, while they let the red and his
companions slip by. This constant reflection
of the blue ray causes the whole air to appear
blue; but what else does it cause? Let us
consider. If air reflects or turns aside, or
hustles out of its place the blue ray, suffering
the rest to pass, it follows as a consequence
that the more air a ray of light encounters,
the more blue will it lose. The sun's rays in
the morning and the evening falling aslant, as
we have said, across a great breadth of our
atmosphere, must lose their blue light to a
terrible extent, and very likely reach us with
the blue all gone, and red lord paramount.
But so, in truth, the case is; and the same fact
which explains the blueness of the atmosphere,