in a direction perpendicular to
the illuminating beam, is also illustrated by the following
experiments: A Nicol's prism, large enough to embrace the entire beam
of the electric lamp, was placed between the lamp and the experimental
tube. A few bubbles of air, carried through the liquid nitrite of
butyl, were introduced into the tube, and they were followed by about
three inches (measured by the mercurial gauge) of air which had passed
through aqueous hydrochloric acid. Sending the polarised beam through
the tube, I placed myself in front of it, my eye 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 now a vivid blue cloud was seen when looked at
downwards or upwards. This truly fine experiments, which I
contemplated making on my own account, was first definitely suggested
by a remark in a letter addressed to me by Professor Stokes.

As regards the polarisation of skylight, the greatest stumbling-block
has hitherto been, that, in accordance with the law of Brewster, which
makes the index of refraction the tangent of the polarising angle, the
reflection which produces perfect polarisation would require to be
made in air upon air; and indeed this led many of our most eminent
men, Brewster himself among the number, to entertain the idea of
aerial molecular reflection. [Footnote: 'The cause of the
polarisation is evidently a reflection of the sun's light upon
something. The question is on what? Were the angle of maximum
polarisation 76 deg., we should look to water or ice as the reflecting
body, however inconceivable the existence in a cloudless atmosphere
and a hot summer's day of unevaporated molecules (particles?) of
wa