ter. But though we were once of this opinion, careful observation
has satisfied us that 90 deg., or thereabouts, is the correct angle, and
that therefore whatever be the body on which the light has been
reflected, if polarised by a single reflection, the polarising angle
must be 45 deg., and the index of refraction, which is the tangent of that
angle, unity; in other words, the reflection would require to be made
in air upon air!' (Sir John Herschel, 'Meteorology,' par. 233.)

Any particles, if small enough, will produce both the colour and the
polarisation of the sky. But is the existence of small
water-particles on a hot summer's day in the higher regions of our
atmosphere inconceivable? It is to be remembered that the oxygen and
nitrogen of the air behave as a vacuum to radiant heat, the
exceedingly attenuated vapour of the higher atmosphere being therefore
in practical contact with the cold of space.]

I have, however, operated upon substances of widely different
refractive indices, and therefore of very different polarising angles
as ordinarily defined, but the polarisation of the beam, by the
incipient cloud, has thus far proved itself to be absolutely
independent of the polarising angle. The law of Brewster does not
apply to matter in this condition, and it rests with the undulatory
theory to explain why. Whenever the precipitated particles are
sufficiently fine, no matter what the substance forming the particles
may be, the direction of maximum polarisation is at right angles to
the illuminating beam, the polarising angle for matter in this
condition being invariably 45 deg..

Suppose our atmosphere surrounded by an envelope impervious to light,
but with an aperture on the sunward side through which a parallel beam
of solar light could enter and traverse the atmosphere. Surrounded by
air not directly illuminated, the track of such a beam would resemble
that of the parallel beam of the electric lamp through an incipient
cloud. The sunbeam would be blue, and it would discharge laterally
light in precisely the same condition as that discharged by the
incipient cloud. In fact, the azure revealed by such a beam would be
to all intents and purposes that which I have called a 'blue cloud.'
Conversely our 'blue cloud' is, to all intents and purposes, an
_artificial sky_.' [Footnote: The opinion of Sir John Herschel,
connecting the polarisation and the blue colour of the sky, is
verified by the foregoing resu