lts. 'The more the subject [the
polarisation of skylight] is considered,' writes this eminent
philosopher, 'the more it will be found beset with difficulties, and
its explanation when arrived at will probably be found to carry with
it that of the blue colour of the sky itself, and of the great
quantity of light it actually does send down to us.' 'We may observe,
too,' he adds, 'that it is only where the purity of the sky is most
absolute that the polarisation is developed in its highest degree, and
that where there is the slightest perceptible tendency to cirrus it is
materially impaired.' This applies word for word to our 'incipient
clouds.']

But, as regards the polarisation of the sky, we know that not only is
the direction of maximum polarisation at right angles to the track of
the solar beams, but that at certain angular distances, probably
variable ones, from the sun, 'neutral points,' or points of no
polarisation, exist, on both sides of which the planes of atmospheric
polarisation are at right angles to each other. I have made various
observations upon this subject which are reserved for the present;
but, pending the more complete examination of the question, the
following facts bearing upon it may be submitted.

The parallel beam employed in these experiments tracked its way
through the laboratory air, exactly as sunbeams are seen to do in the
dusty air of London. I have reason to believe that a great portion of
the matter thus floating in the laboratory air consists of organic
particles, which are capable of imparting a perceptibly bluish tint to
the air. These also showed, though far less vividly, all the effects
of polarisation obtained with the incipient clouds. The light
discharged laterally from the track of the illuminating beam was
polarised, though not perfectly, the direction of maximum polarisation
being at right angles to the beam. At all points of the beam,
moreover, throughout its entire length, the light emitted normally was
in the same state of polarisation. Keeping the positions of the Nicol
and the selenite constant, the same colours were observed throughout
the entire beam, when the line of vision was perpendicular to its
length.

The horizontal column of air, thus illuminated, was 18 feet long, and
could therefore be looked at very obliquely. I placed myself near the
end of the beam, as it issued from the electric lamp, and, looking
through the Nicol and selenite more and more o