Crystals

Considered with reference to their optical properties, there are three cla es: 1. Single refracting crystals, or those belonging to the cubical system. 2. Uniaxial, double refracting. belonging to the rhombohedral or to the tetrahedral system. 3. Biaxial, with two axes of double refraction. In this case both rays are extra ord in xtraordinary rays. In many crystals of the second cla , such as Iceland spar, mica, and tourmaline, the index of refraction for the ordinary is greater than that for the extraordinary ray; such crystals are called "negative" or "repulsive." The reverse is true in some others, as in quartz, ice, zircon; these are "positive" or "attractive."