In the process of measuring the refractive indices of the crystal plate on the crystal-refractometer it was discovered that one of the refractive indexes remains constant at every turn of the glass hemisphere. Another refractive index changes in such a a way that the value of it is 1) maximal 2) minimal. The optical sign of the crystal and the orientation of a plate relative to the optical axis need to be identified.
The plate is cut parallel to the optical axis. 1) The crystal is negative. 2) The crystal is positive.