A Nicol Prism is a device to produce polarized light. It is made from a crystal of calcite, which is cut along a precisely determined plane and then cemented back together with Canada balsam. When a beam of light enters the crystal along the line defined by the mounting of the crystal, it is broken into two beams. In the two beams the planes of vibration of the electric field vector are perpendicular to one another. When the two beams strike the cemented cut, one is passed through and the other is deflected to the side and absorbed in the mounting.