Abstract: In modern optical industry and optical research the anisotropic crystals, such as iceland, KDP, ADP, LiNbO$-3$/, crystalline quartz, etc., have been widely used for making various types of polarizers, optical shutters, interference polarization filters, and light modulators, etc. In order to improve the quality of crystal elements, the accuracy of crystal optical axis orientation must be improved. In this paper a new type of instrument is described for determining the perpendicular orientation of the crystal optical axis of a crystal plate to the plate surface. A converging bundle of polarized rays passes through the plate and forms a set of ring interferogram and a dark cross image in the interferogram. As the working stage is rotated, the center of the cross and the rings move along a circle, if the surface of the plate is not perpendicular to the optical axis. The accuracy of data read directly from a beeswax screen of $phi 150 mm does not exceed six arc minutes. If the data and other parameters are input to a microcomputer IBM/PC to remove the theoretical deviations of the instrument the accuracy of two arc minutes can be obtained. The size of crystal under test can be $phi 100 $MUL (0.5 - 100) mm. Theoretical calculation shows that the accuracy of a thick crystal plate under test and the data read from an interferential ring of lower order of interference are improved. In this instrument the crystal cone interferogram, clean and bright, is projected on a screen, and it is suitable for teaching demonstration and shop testing. The cost of this instrument is lower because of its simple structure. !2
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