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system of splendidly-colored rings. Precisely the same
phenomena are observed when we look at the blue firma—
ment in a direction perpendicular to the solar rays.

We have thus far illuminated our artificial sky with
ordinary light. We will now examine the effects produced
when the light which illuminates the particles is itself
polarized. In front of the electric lamp, and between it
and the experimental tube, is placed this fine N icol’s prism,
whiCh is sufficiently large to embrace and to polarize the
entire beam. The plane of vibration of the light now
emergent from the prism, and falling upon the cloud, is
vertical; and we find that this formless aggregate of infini-
tesimal particles, without definite structure, is absolutely
incompetent to scatter the light upward or downward,
while it freely discharges the light horizontally, right and
left. I turn the polarizing Nicol so as to render the plane
of vibration horizontal; the cloud now freely scatters the
light vertically upward and downward, but it is absolutely
incompetent to shed a ray horizontally to the right or left.

Suppose the atmosphere of our planet to be surrounded
by an envelope impervious to light, with an aperture on
the sunward side, through which a solar beam could enter.
Surrounded on all sides by air not directly illuminated, the
track of the sunlight would resemble that of the electric
beam in a dark space filled with our incipient cloud. The
course of the sunbeam would be blue, and it would dis-
charge laterally, in all directions round it, light in precisely
the same polarized condition as that discharged from the
incipient cloud. In fact, the azure revealed by the sunbeam
would be the azure of such a cloud. And if, instead of
permitting the ordinary light of the sun to enter the aper-
ture, a N icol’s prism were placed there, which should
polarize the sunlight on its entrance into our atmOSphere,
the particles producing the color of the sky would act
precisely like those of our incipient cloud. In two directions

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