Doubly refracting crystals, such as calcite and quartz, break up light rays into two parts, called ordinary rays and extraordinary rays, which are polarized at right angles to each other. Such a crystal has a different refractive index for each of the two rays, and they are bent at different angles when they enter the crystal. This double refraction will form two images when a calcite crystal is placed over a dot on a piece of paper. The dot appears as two dots a small distance apart. Rotating the crystal causes one of the dots to rotate about the other. The dot that remains stationary is the image formed by the ordinary ray. This always lies in the plane of incidence (plane including the normal and the incident ray). The moving dot is the image formed by the extraordinary ray. [pg. 55]
[Light and Color, by Clarence Rainwater, Prof. of Physics, San Francisco State College, Original Project Editor Herbert S. Zim, Golden Press, NY, Western Publishing Company, Inc., 1971.]