appears of a blue-black colour, the blue reflected from the
comparatively small amount of dust particles being seen against the
intense black of stellar space. It is for the same reason that the
"Italian skies" are of so rich a blue, because the Mediterranean Sea on
one side and the snowy Alps on the other do not furnish so large a
quantity of atmospheric dust in the lower strata of air as in less
favorably situated countries, thus leaving the blue reflected by the
more uniformly distributed fine dust of the higher strata undiluted. But
these Mediterranean skies are surpassed by those of the central Pacific
ocean, where, owing to the small area of land, the lower atmosphere is
more free from coarse dust than in any other part of the world.

If we look at the sky on a perfectly fine summer's day, we shall find
that the blue colour is the most pure and intense overhead, and when
looking high up in a direction opposite to the sun. Near the horizon it
is always less bright, while in the region immediately around the sun it
is more or less yellow. The reason of this is that near the horizon we
look through a very great thickness of the lower atmosphere, which is
full of the larger dust particles reflecting white light, and this
dilutes the pure blue of the higher atmosphere seen beyond. And in the
vicinity of the sun a good deal of the blue light is reflected back into
space by the finer dust, thus giving a yellowish tinge to that which
reaches us reflected chiefly from the coarse dust of the lower
atmosphere. At sunset and sunrise, however, this last effect is greatly
intensified, owing to the great thickness of the strata of air through
which the light reaches us. The enormous amount of this dust is well
shown by the fact that, then only, we can look full at the sun, even
when the whole sky is free from clouds and there is no apparent mist.
But the sun's rays then reach us after having passed, first, through an
enormous thickness of the higher strata of the air, the minute dust of