Note 44 — Chapter III · Investigation of Microscopic and Optical Characters of Minerals.

The lower nicol is generally so adjusted that its plane of vibration is parallel to the north and south cross-wire in the eye-piece. This adjustment can be tested by means of a section of biotite, showing cleavage cracks. When the plane of vibration of the polarizer is parallel to the N. and S. cross-wire in the eye-piece, the biotite section becomes almost dark when its cleavage cracks are parallel to the same cross-wire. The upper nicol, or analyzer, must, of course, be removed during this test. This method is more convenient than taking the nicol out of its frame, in order to ascertain its plane of vibration (the direction of its shorter diagonal).