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The radiations emitted by a focus tube are filtered through a sheet of aluminium foil or a screen of black paper, in order to eliminate the luminous rays which might accompany them. While studying these radiations by means of their action on a small spark, I discovered that they are plane-polarized as soon as emitted. I further proved that when these radiations traverse a plate of quartz in a direction at right angles to its axis, or a lump of sugar, their plane of action undergoes a rotation just like the plane of polarization of a pencil of light (see pp. 5 and 6).

I then asked myself if a rotation could also be obtained by passing the radiations of the focus tube through a pile of Reusch mica sheets. I observed, in fact, a rotation of from 25° to 30° in the same direction as that of polarized light. This action of a pile of micas made me at once infer that a single sheet of mica must act, and that this action must be depolarization, or, rather, the production of elliptic polarization; this is