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Under certain circumstances, it may be quenched by a
Nicol’s prism, and we then obtain the true color of the grass
and foliage. Trees and meadows thus regarded exhibit a
richness and softness of tint which they never show as long
as the superficial light is permitted to mingle with the true
interior emission. The needles of the pines show this effect
very well, large-leaved trees still better; while a glimmer-
ing field of maize exhibits the most extraordinary variations
when looked at through the rotating Nicol.

Thoughts and questions like those here referred to took
me, in August, 1869, to the top of the Aletschhorn. The
effects described in the foregoing paragraphs were for the
most part reproduced in the summit of the mountains I
scanned the whole of the sky with my Nicol. Both alone
and in conjunction with the selenite it pronounced the per-
pendicular to the solar beams to be the direction of maxi-
mum polarization. But at no portion of the firmament was
the polarization complete. The artificial sky produced in
the experiments recorded in the preceding discourse could,
in this respect, be rendered more perfect than the natural
one ; while the gorgeous “ residual blue ” which makes its
appearance when the polarization of the artificial sky ceases
to be perfect, was strongly contrasted with the lack-lustre
hue which, in the case of the firmament, outlived the ex-
tinction of the brilliance. With certain substances, how-
ever, artificially treated, this dull residue may also be ob-

All along the are from the Matterhorn to Mont Blanc
the light of the sky immediately above the mountains was
powerfully acted upon by the Nicol. In some cases the
variations of intensity were astonishing. I have already
said that a little practice enables the observer to shift the
Nicol from one position to another so rapidly as to render
the alternate extinction and restoration of the light imme-
diate. When this was done along the arc to which I have

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