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“This is a very mysterious and a very beautiful phenomenon when
observed by the aid of a polaris00pe, consisting of a tourmaline plate,
with a slice of Iceland crystal or nitre, cut at right-angles to the Optic
axis, and applied on the side of the tourmaline farthest from the eye. In
a cloudless day, if the sky be explored in all parts by looking through
this compound plate, the polarized rings will be seen developed with more
or less intensity in every region but that nearest the sun and that most
distant from it—the maximum of polarization taking place on a zone of
the sky 90° from the sun, or in a great circle, having the sun for one of
its poles, so that the cause of polarization is evidently a reflection of the
sun’s light on something. The question is, on what? Were the angle of
maximum polarization ’7 6° we should look to water or ice for the reflect-
ing body. But though we were once of this Opinion (art. Light, Encycl.
Metropol. § 1143), careful observation has satisfied us that 90°, or there-
abouts, is the correct angle, and that therefore, whatever be the body on
which the light has been reflected, if polarized by a single reflection, the
‘ polarizing angle ’ must be 45°, and the index of refraction, which is the
tangent of that angle, unity; in other words, the reflection would require
to be made in air upon air! The only imaginable way in which this could
happen would be at the plane of contact of two portions of air differently
heated, such as might be supposed to occur at almost every point of the
atmOSphere in a bright sunny day ; but against this there seems to be an
insuperable objection. The polarization is most regular and complete, as
we have lately been able to satisfy ourselves under the most favorable
possible atmospheric conditions, after sunset, in the bright twilight of a
summer night, with the sun some degrees below the horizon, and long
after all the tremor and turmoil of the air, due to irregular heating, must
have completely subsided. On the other hand, if effected by several suc-
cessive reflections, what is to secure a large majority of them being in
one plane (in which case only their polarizing effect would accumulate);
and of those which become ultimately efl’ective, what. is there to deter-
mine an ultimate deviation of 90° as that of the maximum? The more
the subject is considered, the more it will be found beset with difficulties;
and its explanation, when arrived at, will probably be found to carry

with it that of the blue color of the sky itself.”
SIR JOHN HERSCHEL.

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