Determination of the optical sign of a crystal by a conoscopic method

To determine the optical sign of an investigated crystal plate, a conoscopic pattern of the latter is changed in an optical system without a polarizing microscope. To change the conoscopic pattern, an auxiliary plane-parallel crystal plate of arbitrary thickness and known optical sign is used. Such a plate, which is either capable of rotation about its vertical axis or fixed at a small angle to the investigated plate, is introduced into the optical system.  Upon rotation of the auxiliary crystal plate, isochromatic rings on the conoscopic pattern are offset. Based on the direction of the offset from the centre of the pattern, a conclusion is drawn about the optical sign of the investigated crystal plate. When the auxiliary crystal plate is fixed at a small angle to the investigated plate, either two or three conoscopic patterns are observed on the screen. The optical sign of the investigated crystal plate is determined by the number of conoscopic patterns on the screen. If there are three patterns, then the optical signs of the auxiliary and the investigated plates are the same. If there are two patterns, then the optical signs of the auxiliary and the investigated plates are opposite.