To determine an optical sign of the investigated crystal plate, a conoscopic pattern of the latter is changed in an optical
system without a polarizing microscope. Unknown optical sign of the crystal plate is determined by the nature of
changes made to its conoscopic pattern by the plate of known optical sign. To change the conoscopic pattern, an
auxiliary plane-parallel crystal plate of arbitrary thickness and known optical sign is used. Such plate either capable of
rotation about its vertical axis is introduced to the optical system.