Page:Collected Physical Papers.djvu/114

94 The analyser and the polariser are now exactly crossed, and there is not the slightest action on the receiver. The polariser is now slightly displaced from the crossed position, and the galvanometer-spot is seen to be violently deflected.

The gratings are once more adjusted in a crossed position. I have in my hand a large block of the crystal beryl which is perfectly opaque to light. I now hold the crystal with its principal plane inclined at 45° between the crossed gratings, and the galvanometer-spot, hitherto quiescent, sweeps across the scale. It is very curious to observe the restoration of the extinguished field of electric radiation, itself invisible, by the interposition of what appears to the eye to be a perfectly opaque block of crystal. If the crystal is slowly rotated, there is no action on the receiver when the principal plane of the crystal is parallel to either the polariser or the analyser. Thus, during one complete rotation there are four positions of the crystal when no depolarisation effect is produced.

Rotation of the crystal, when held with its optic axis parallel to the incident ray, produces no action. The field remains dark.

Here is another large crystal, idocrase, belonging to the orthorhombic system, which shows the same action. It is not at all necessary to have large crystals; a piece of calc-spar, taken out of an optical instrument, will be found effective in polarising the electric ray. The effect produced by the crystal epidote is, however, extraordinary. I have here a piece with a thickness of only ·7 cm., smaller than a single wave-length of electric radiation, and yet observe how strong is its depolarising effect.