Note 54 — Chapter III · Investigation of Microscopic and Optical Characters of Minerals.

The ¼ undulation mica plate consists of a thin cleavage of mica on which is marked c, the vibration direction of the slower ray, which in mica is the line joining the “optic axes.” The thickness is such that the slower ray is ¼ wave-length behind the faster and the interference color is a bluish-gray. The gypsum plate is a thin cleavage of gypsum, on which is usually marked a, the vibration direction of the faster ray. The chosen thickness is such as to produce the red interference color of the 1° order.