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phere and a hot summer day, of unevaporated particles of
water.” But a polarizing angle of 45° corresponds to a
refractive index of 1 ; this means that there is no refraction
at all, in which case we ought to have no reflection. Brew-
ster, therefore, and others came to the conclusion that the
reflection was from the particles of air themselves. Dr.
Rubenson, of Upsala, made the angle enclosed between the
direct and reflected beams 90° 2'; “ the half of which,” says
Mr. Buchan, in his excellent 'little “Handy Book of Me—
teorology,” “is so near the polarizing angle of air as to leave
no doubt that the light of the sky, as first stated by Brew-
ster, is polarized by reflection from the particles of air.”

If you doubt the wisdom, acknowledge, at all events,
the faith in your capacity which has caused me to bring
so entangled a subject before you. I would fain'believe,
however, that even the intellect which draws its culture
from a totally different source, may have its interest excited
in subjects like the present, dark and diflicult though they
seem. I do not expect that you will grasp all the details
of this discussion ; but everybody present will, I think, see
the extremely important part hitherto played by the law
of Brewster in speculations as to the color and polarization
of the sky. Let me now endeavor to demonstrate in your
presence, firstly, and in confirmation of our former experi-
ments, that sky-blue may be produced by exceedingly mi-
nute particles of any kind of matter; secondly, that polari-
zation identical with that of the sky is produced by such
particles; and thirdly, that matter in this fine state of di-
vision, where its particles are small in comparison with the
height and span of a wave of light, releases itself completely
from the law of Brewster; the direction of maximum polari-
zation being absolutely independent of the polarizing angle
as hitherto defined. "

Into this experimental tube, in the manner already de-
scribed, I introduce a vapor which is decomposable by the

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