axis of the
illuminating beam.

It is exceedingly interesting to observe both the growth and the decay
of this polarization. For ten or fifteen minutes after its first
appearance, the light from a vividly illuminated incipient cloud,
looked at horizontally, is absolutely quenched by a Nicol prism with
its longer diagonal vertical. But as the sky-blue is gradually
rendered impure by the introduction of particles of too large a size,
in other words, as real clouds begin to be formed, the polarization
begins to deteriorate, a portion of the light passing through the
prism in all its positions, as it does in the case of skylight. It is
worthy of note that for some time after the cessation of perfect
polarization the _residual_ light which passes, when the Nicol is in
its position of minimum transmission, is of a gorgeous blue, the
whiter light of the cloud being extinguished. When the cloud-texture
has become sufficiently coarse to approximate to that of ordinary
clouds, the rotation of the Nicol ceases to have any sensible effect
on the light discharged at right angles to the beam.

The perfection of the polarization in a direction perpendicular to the
illuminating beam may be also illustrated by the following experiment,
which has been executed with many vapours. A Nicol prism large enough
to embrace the entire beam of the electric lamp was placed between the
lamp and the experimental tube. Sending the beam polarized by the
Nicol through the tube, I placed myself in front of it, the eyes being
on a level with its axis, my assistant occupying a similar position
behind the tube. The short diagonal of the large Nicol was in the
first instance vertical, the plane of vibration of the emergent beam
being therefore also vertical. As the light continued to act, a superb
blue cloud visible to both my assistant and myself was slowly formed.
But this cloud, so deep and rich when looked at from the positions
mentioned, utterly disappeared when looked at vertically downwards,
or vertically upwards. Reflection from the cloud was not possible in
these directions. When the large Nicol was slowly turned round its
axis, the eye of the observer being on the level of the beam, and the
line of vision perpendicular to it, entire extinction of the light
emitted horizontally occurred when the longer diagonal of the large
Nicol was vertical. But a vivid blue cloud was seen when looked at
downwards or upwards. This truly fine experiment, which