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not, be able to overcome this difficulty, and to reach to-
gether a greater elevation.



THE vision of an object always implies a differential
action on the retina of the observer. ,The object is dis-
tinguished from surrounding space by its excess or defect
of light in relation to that space. By altering the illumi-
nation, either o-f the object itself or of its environment, we
alter the appearance of the object. Take the case of clouds
floating in the atmosphere with patches of blue between
them. Any thing that changes the illumination of either
alters the appearance of both, that appearance depending,
as stated, upon differential action. Now the light of the
sky being polarized, may, as the reader of the foregoing
pages knows, be in great part quenched by a Nicol’s
prism, while the light of a cloud, being unpolarized, cannot
be thus extinguished. Hence the possibility of very re-
markable variations, not only in the aspect of the firma-
ment, which is really. changed, but also in the aspect of the
clouds which have that firmament as a background. It is
possible, for example, to choose clouds of such a depth of
shade that when the Nicol quenches the light behind them,
they shall vanish, being undistinguishable from the residual
dull tint which outlives the extinction of the brilliance of
the sky. A cloud less deeply shaded, but still deep enough,
when viewed with the naked eye, to appear dark on a
bright ground, is suddenly changed to a white cloud on a
dark ground by the quenching of the sky behind it. When
a reddish cloud at sunset chances to float in the region of
maximum polarization, the quenching of the sky behind it
causes it to flash with a brighter crimson. Last Easter eve
the Dartmoor sky, which had just been cleansed by a snow-

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