We have seen that the air near the earth's surface is full of rather
coarse particles which reflect all the rays, and which therefore produce
no one colour. But higher up the particles necessarily become smaller
and smaller, since the comparatively rare atmosphere will support only
the very smallest and lightest. These exist throughout a great thickness
of air, perhaps from one mile to ten miles high or, even more, and blue
or violet rays being reflected from the innumerable particles in this
great mass of air, which is nearly uniform in all parts of the world as
regards the presence of minute dust particles, produces the constant and
nearly uniform tint we call sky-blue. A certain amount of white or
yellow light is no doubt reflected from the coarser dust in the lower
atmosphere, and slightly dilutes the blue and renders it not quite so
deep and pure as it otherwise would be. This is shown by the increasing
depth of the sky-colour when seen from the tops of lofty mountains,
while from the still greater heights attained in balloons the sky
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.
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