amined is now interposed with its principal plane inclined at 45° to the horizon.
Rhombohedral System
(1) Beryl.—The first piece experimented on was a large crystal of Beryl. It is a hexagonal prism with basal planes. The specimen examined has each face 11 × 5 cm. The three axes lying in the same plane are inclined at 60° to each other, the fourth axis, which is also the optical axis, is at right angles to the plane containing the other three. The crystal was optically opaque.
On interposing this block with its principal plane inclined at 45°, the galvanometer spot flew off the scale. The crystal had thus produced the well-known depolarising action. The crystal was now turned round till its principal plane coincided with the vibration plane of the Polariser. There was now no action on the galvanometer. On continuing the rotation, the galvanometer at once responded. The spot became quiescent a second time, when the principal plane coincided with the plane of vibration of the Analyser.
The crystal was now placed with its optical axis parallel to the direction of the incident ray. There was now no action on the galvanometer. Rotation of the crystal round this axis did not produce any effect on the galvanometer. The field continued to be dark.
The piece of Beryl used in the above experiment was unusually large. But in the following experiments the crystals were quite small.
(2)Apatite.—In repeating the experiment with this crystal, strongly marked double refraction effect was exhibited.
