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THE SKY OF THE ALPS. 269

mountain sprang forth with astonishing solidity and detach-
ment from the surrounding air. The changes of the Dom
were still more wonderful. A vast amount of light could
be removed from the sky behind it, for it occupied the po-
‘sition of maximum polarization. By a little practice with
the Nicol it was easy to render the extinction of the light,
0r its restoration, almost instantaneous. When the sky was
quenched, the four minor peaks and buttresses, and the
summit of the Dom, together with the shoulder of the Al-
phubel, glowed as if set suddenly on fire. This was imme-
diately dimmed by turning theNicol through an angle of
90°. It was not the stoppage of the light of the sky behind
the mountains alone which produced this startling effect;
the air between them and me was highly opalescent, and
the quenching of this intermediate glare augmented re-
markably the distinctness of the mountains.

On the morning of August 24th similar effects were fine-
ly shown. . At 10 A. M. all three mountains, the Dom, the
Matterhorn, and the Weisshorn, were powerfully affected
by the Nicol. But in this instance also the line drawn to
the Dom being accurately perpendicular to the direction of
the solar shadows, and consequently very nearly perpen-
dicular to the solar beams, the effects on this mountain were
most striking. The gray summit of the Matterhorn at the
same time could scarcely be distinguished from the Opalesw
cent haze around it; but when the Nicol quenched the
haze, the summit became instantly isolated, and stood out
in bold definition. It is to be remembered that in the pro-
duction of these effects the only things changed are the
sky behind and the luminous haze in front of the moun-
tains; that these are changed because the light emitted
from the sky and from the haze is plane polarized light,”l
and that the light from the snows and from the mountains
being sensibly unpolarized, is not directly affected by the

1 Defined at page 255.

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