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position it is transmitted. Our way is now, to some extent,
cleared toward an examination of the light of the sky.
Looking at various points of the blue firmament through a
N icol’s prism, and turning the prism round its axis, we im-
mediately notice variations of brightness. In certain posi-
tions of the prism, and from certain points of the firmament,
the light appears to be freely transmitted ; while it is only
necessary to turn the prism round its axis through an angle
of 90° to materially diminish the intensity of the light. On
close scrutiny it is found that the difference produced by
the rotation of the prism is greatest when the sky is re-
garded in a direction at right angles to that of the solar
rays through the air.

Let me describe a few actual observations made some
days ago on Primrose Hill. The sun was near setting, and
a few scattered neutral-tint clouds, which failed to catch
the dying light, were floating in the air. When these were
looked at across the track of the solar beams, it was pos-
sible, by turning the' Nicol round, to see them either as
white clouds on a dark ground, or as dark clouds on a bright
ground.1 In certain positions of the prisms the sky-light
was in great part quenched, and then the clouds, projected
against the darkness of space, appeared white. Turning
the Nicol 90° round its axis, the brightness of the sky Was
restored, the clouds becoming dark through contrast with
this brightness. Experiments of this kind prove that the
blue light sent to. us by the firmament is polarized, and that
the direction of most perfect polarization is perpendicular
to the solar rays. Were the heavenly azure like the light
scattered from a thick cloud, the turning of the prism would
have no effect upon it; it would be transmitted equally dur-
ing the entire rotation of the prism. The light of the sky is
in great part quenched, because it is in great part polarized.

1 I was not aware when these words were written that this observation
was made by the indefatigable Brewster.

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