I say that the blue which is seen in the
atmosphere is not its own color, but is
caused by the heated moisture having
evaporated into the most minute
imperceptible particles, which the
beams of the solar rays attract and
cause to seem luminous against the
deep intense darkness of the region ...
above them. And this may be seen, as
I myself saw it, by anyone who ascends
Mon Boso,* a peak of the Alps that
divides France from Italy ... And I saw
the atmosphere dark overhead, and the
rays of the sun striking the mountain
had far more brightness than in the
plains below, because less thickness of
atmosphere lay between the summit
of this mountain and the sun.