However, in these cases it becomes important to also be sure of the " optic sign " of the mineral ( essentially, this tells you how the optic angle is orientated with respect to the whole optical indicatrix describing the refractive indices of the mineral in 3D ).
2.
More simply, in ordinary cross polarized light, the plate can be used to distinguish the orientation of the optical indicatrix relative to crystal elongation-that is, whether the mineral is " length slow " or " length fast "-based on whether the visible interference colors increase or decrease by one order when the plate is added.
मोबाइल