ere not much diminished in brilliancy. I should estimate the
distance across the valley, as the crow flies, to the opposite
mountain, at nine miles; so that a body of air of this thickness can,
under favourable circumstances, produce chromatic effects of
polarisation almost as vivid as those produced by the sky itself.

Again: the light of a landscape, as of most other things, consists of
two parts; the one, coming purely from superficial reflection, is
always of the same colour as the light which falls upon the landscape;
the other part reaches us from a certain depth within the objects
which compose the landscape, and it is this portion of the total light
which gives these objects their distinctive colours. The white light
of the sun enters all substances to a certain depth, and is partly
ejected by internal reflection; each distinct substance absorbing and
reflecting the light, in accordance with the laws of its own molecular
constitution. Thus the solar light is _sifted_ by the landscape, which
appears in such colours and variations of colour as, after the sifting
process, reach the observer's eye. Thus the bright green of grass, or
the darker colour of the pine, never comes to us alone, but is always
mingled with an amounts of light derived from superficial reflection.
A certain hard brilliancy is conferred upon the woods and meadows by
this superficially-reflected light. Under certain circumstances, it
may be quenched by a Nicol's prism, and we then obtain the true colour
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 glimmering 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 on the summit
of the mountain. I scanned the whole of the sky with my Nicol. Both
alone, and in conjunction with the selenite, it pronounced the
perpendicular to the solar beams to be the direction of maximum
polarisation.

But at no portion of the firmament was the polarisation complete. The
artificial sky produced in the experiments r