it also underwent striking changes. In one position
of the prism it exhibited a pale white against a dark background; in
the rectangular position it was a dark mauve against a light
background. The red of the Matterhorn changed in a similar manner;
but the whole mountain also passed through wonderful changes of
definition. The air at the time was filled with a silvery haze, in
which the Matterhorn almost disappeared. This could be wholly
quenched by the Nicol, and then the mountain sprang forth with
astonishing solidity and detachment from the surrounding air. The
changes of the Dom were still more wonderful. A vast amounts of light
could be removed from the sky behind it, for it occupied the position
of maximum polarisation. By a little practice with the Nicol it was
easy to render the extinction of the light, or 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 Alphubel, glowed as if set suddenly on fire. This was immediately
dimmed by turning the Nicol through an angle of 90 deg.. 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
remarkably the distinctness of the mountains.

On the morning of August 24 similar effects were finely 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 very nearly perpendicular to the solar
beams, the effects on this mountain were most striking. The grey
summit of the Matterhorn, at the same time, could scarcely be
distinguished from the opalescent 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 production of
these effects the only things changed are the sky behind, and the
luminous haze in front of the mountains; that these are changed
because the light emitted from the sky and from the haze is plane
polarised light, and that the light from the snows and from the
mountains, being sensibly unpolarised, is not directly affected by the
Nicol. It will also be understood that it is not the interposition of
the haze _as an opaque body_ that renders the mountains