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STRUCTURE AND LIGHT OF THE SKY. 265

showed the green at its maximum. At a further distance
of 45° from the position of maximum red, the color disap-
pears a second time. We have there a second neutral
point, beyond which the green comes again into view, at-
taining its maximum brillianey at the end of a rotation of
180°. By the rotation of the Nicol, therefore, through an
angle of 90°, we produce a color complementary to that
with which we started.

As may be inferred from this result, the selenite ring-
system changes its character when the Nicol is turned.
It is possible to have the centre of the circle dark, the
surrounding rings being vividly colored.- The turning of
the Nicol through an angle of 90° renders the centre bright,
while every point occupied by a certain color in the first
instance is occupied by the complement of that color in the
second. By curious internal actions, not here to be de-
scribed, the cloud in our experimental tube sometimes
divides itself into sections of different textures. Some sec-
tions are coarser than others, while it often happens that
some are iridescent to the naked eye, and others not.
Looking" normally at such a cloud through the selenite and
Nicol, it often happens that in passing from section to sec-
tion the whole character of the ring-system is changed.
You start with a section producing a dark centre and a
corresponding system of rings; you pass through a neutral
point to another section and find there the centre bright,
and each of the first rings displaced by one of the comple-
mentary color. Sometimes as many as four such rever-
sions occur in the cloud of an experimental tube a yard
long. Now, the changes here indicated mean that in passing
from section to section of the cloud the plane of vibration
of the polarized light turns suddenly through an angle of
90°; this change being entirely due to the different texture
of the two parts of the cloud.

You will now be able to understand, as far as it is ca-
12

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