rtz show different colours.
The magnet produces an effect equivalent to this rotation. The
puce-coloured circle is now before you on the screen. (See fig. 44,
where N is the nozzle of the lamp, H the first Nicol, Q the biquartz
plate, L a lens, M the electro-magnet, with the heavy glass across its
perforated poles, and P the second Nicol.) Exciting the magnet, one
half of the image becomes suddenly red, the other half green.
Interrupting the current, the two colours fade away, and the primitive
puce is restored.

The action, moreover, depends upon the polarity of the magnet, or, in
other words, on the direction of the current which surrounds the
magnet. Reversing the current, the red and green reappear, but they
have changed places. The red was formerly to the right, and the green
to the left; the green is now to the right, and the red to the left.
With the most exquisite ingenuity, Faraday analyzed all those actions
and stated their laws. This experiment, however, long remained a
scientific curiosity rather than a fruitful germ. That it would bear
fruit of the highest importance, Faraday felt profoundly convinced,
and present researches are on the way to verify his conviction.

[Illustration: Fig. 44]

Sec. 9. _Iris-rings surrounding the Axes of Crystals._

A few more words are necessary to complete our knowledge of the
wonderful interaction between ponderable molecules and the ether
interfused among them. Symmetry of molecular arrangement implies
symmetry on the part of the ether; atomic dissymmetry, on the other
hand, involves the dissymmetry of the ether, and, as a consequence,
double refraction. In a certain class of crystals the structure is
homogeneous, and such crystals produce no double refraction. In
certain other crystals the molecules are ranged symmetrically round a
certain line, and not around others. Along the former, therefore, the
ray is undivided, while along all the others we have double
refraction. Ice is a familiar example: its molecules are built with
perfect symmetry around the perpendiculars to the planes of freezing,
and a ray sent through ice in this direction is not doubly refracted;
whereas, in all other directions, it is. Iceland spar is another
example of the same kind: its molecules are built symmetrically round
the line uniting the two blunt angles of the rhomb. In this direction
a ray suffers no double refraction, in all others it does. This
direction of no double refraction is ca