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STRUCTURE AND LIGHT OF THE SKY. 257

When a luminous beam impinges at the proper angle
on a plane-glass surface it is polarized by reflection. It is
polarized, in part, by all oblique reflections; but at one
particular angle, the reflected light is perfectly polarized.
An exceedingly beautiful and simple law, discovered by
Sir David Brewster, enables us readily to find the polarizing
angle of any substance whose refractive index is known.
This law was discovered experimentally by Brewster; but
the Wave Theory of light renders a complete reason for
the law. A geometrical image of it is thus given: When
a beam of light impinges obliquely upon a plate of glass it
is in part reflected and in part refracted. At one particular
incidence the reflected and the refracted portions of the
beam are at right angles to each other. The angle of inci-
dence is then the polarizing angle. It varies with the re-
fractive index of the substance; being for water 52%, for
glass 57%, and for diamond 68°.

It has been already stated that, in order to obtain the
most perfect polarization of the firmamental light, the sky
must be regarded in a direction at right angles to the solar
beams. This is sometimes expressed by saying that the
place of maximum polarization is at an angular distance of
90° from the sun. This angle, enclosed as it is between the
direct and reflected rays, comprises both the angles of inci-
dence and reflection, supposing the polarization to be due
to a single reflection. Hence the angle of incidence is half
of 90°, or 45°. This is the atmospheric polarizing angle,
and the question is, what known substance possesses an
index of refraction to correspond with this polarizing angle ?
“ If,” says Sir John Herschel, “ we knew this substance, we
might be, tempted to conclude that particles of it, scattered
in the atmosphere, produce the polarization of the sky.
Were the angle of maximum polarization 7 6° (instead of
90°), we should look to water or ice, as the reflecting body,
however inconceivable the existence in a cloudless atmoso

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