or the analyzer through the smallest angle, the uniformity of the colour disappears, and the two halves of the quartz 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 called the optic axis of the crystal.