ng through this compound
and highly attenuated atmosphere the beam of the electric light,
within the tube arises gradually a splendid azure, which strengthens
for a time, reaches a maximum of depth and purity, and then, as the
particles grow larger, passes into whitish blue. This experiment is
representative, and it illustrates a general principle. Various other
colourless substances of the most diverse properties, optical and
chemical, might be employed for this experiment. The _incipient
cloud_, in every case, would exhibit this superb blue; thus proving to
demonstration that particles of infinitesimal size, without any colour
of their own, and irrespective of those optical properties exhibited
by the substance in a massive state, are competent to produce the blue
colour of the sky.

Sec. 13. _Polarization of Skylight_.

But there is another subject connected with our firmament, of a more
subtle and recondite character than even its colour. I mean that
'mysterious and beautiful phenomenon,' as Sir John Herschel calls it,
the polarization of the light of the sky. Looking at various points of
the blue firmament through a Nicol prism, and turning the prism round
its axis, we soon notice variations of brightness. In certain
positions of the prism, and from certain points of the firmament, the
light appears to be wholly transmitted, while it is only necessary to
turn the prism round its axis through an angle of ninety degrees to
materially diminish the intensity of the light. Experiments of this
kind prove that the blue light sent to us by the firmament is
polarized, and on close scrutiny it is also found that the direction
of most perfect polarization is perpendicular to the solar rays. Were
the heavenly azure like the ordinary light of the sun, the turning of
the prism would have no effect upon it; it would be transmitted
equally during the entire rotation of the prism. The light of the sky
may be in great part quenched, because it is in great part polarized.

The same phenomenon is exhibited in perfection by our actinic clouds,
the only condition necessary to its production being the smallness of
the particles. In all cases, and with all substances, the cloud formed
at the commencement, when the precipitated particles are sufficiently
fine, is _blue_. In all cases, moreover, this fine blue cloud
polarizes _perfectly_ the beam which illuminates it, the direction of
polarization enclosing an angle of 90 deg. with the