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STRUCTURE AND LIGHT OF THE SKY. 261

obliquely to it are transmitted. It is easy, therefore, to see
that, from the position in which the prism must be held to
transmit or to quench the light of our incipient cloud, we
can infer the direction of the Vibrations of that light. You
will be able to picture those Vibrations without difficulty.
Suppose a line drawn from any point of the “ cloud ” per-
pendicular to the illuminating beam. The particles of ether
along that line, which carry the light from the cloud to the
eye, vibrate in a direction perpendicular both to the line
and to the beam. And if any number of lines be drawn
in the same way from the cloud, like the spokes of a wheel,
the particles of ether along all of them oscillate in the same
manner. Wherefore, if a plane surface be imagined cutting
the incipient cloud at right angles to its length, the vibra-
tions discharged laterally are all parallel to this surface.
This is the plane of Vibration of the polarized light.

Our incipient blue cloud is a Virtual N icol’s prism, and,
between. it and the real prism, we can produce all the
effects obtainable between the polarizer and analyzer of a
polariscope. When, for example, a thin plate of selenite,
which is crystallized sulphate of lime, is placed between
the Nicol and the incipient cloud, we obtain the splendid
chromatic phenomena of polarized light. The color of the
gypsum-plate, as many of you know, depends upon its
thickness. If this be uniform, the color is uniform. If, on
the contrary, the plate be wedge-shaped, thickening grad-
ually and uniformly from edge to back, we have brilliant
bands of color produced parallel to the edge of the wedge.
Perhaps the best form of plate for experiments of this
character is that now in my hand, which was prepared for
me some years ago by a man of genius in his way, the late
Mr. Darker, of Lambeth. It consists of a plate of selenite
thin at the centre, and gradually thickening toward the
circumference. Placing this film between the Nicol and
the cloud, we obtain, instead of a series of parallel bands, a

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