ecorded in the preceding
pages could, in this respect, be rendered far more perfect than the
natural one; while the gorgeous 'residual blue' which makes its
appearance when the polarisation 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 extinction of the brilliancy.
With certain substances, however, artificially treated, this dull
residue may also be obtained.

All along the arc 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
alternative extinction and restoration of the light immediate. When
this was done along the arc to which I have referred, the alternations
of light and darkness resembled the play of sheet lightning behind the
mountains. There was an element of awe connected with the suddenness
with which the mighty masses, ranged along the line referred to,
changed their aspect and definition under the operation of the prism.

*****

The physical reason of the blueness of both natural and artificial
skies is, I trust, correctly given in the essay on the Scientific use
of the Imagination published in the second volume of these Fragments.

********************

V. ON DUST AND DISEASE.

[Footnote: A discourse delivered before the Royal Institution of Great
Britain, January 21, 1870.]

Experiments on Dusty Air.

SOLAR light, in passing through a dark room, reveals its track by
illuminating the dust floating in the air. 'The sun,' says Daniel
Culverwell, 'discovers atomes, though they be invisible by
candle-light, and makes them dance naked in his beams.'

In my researches on the decomposition of vapours by light, I was
compelled to remove these 'atoms' and this dust. It was essential that
the space containing the vapours should embrace no visible thing--that
no substance capable of scattering light in the slightest sensible
degree should, at the outset of an experiments, be found in the wide
'experimental tube' in which the vapour was enclosed.

For a long time I was troubled by the appearance there of floating
matter, which, though invisible in diffuse daylight, was at once
revealed by a powerfully condensed beam. Two U-tubes were