= Solution
On rotating a fixed crystallite around the microscope axis, the angle $\alpha$ to the fixed <polarizer> changes. The factor $\sin^2(2\alpha)$ repeats every90degrees. Each birefringent region is therefore extinguished four times in a complete revolution and reaches maximum brightness45degrees from its own extinction positions.
Different regions need not extinguish together if their principal in-plane directions differ. For a fixed propagation direction through a uniform region, azimuthal rotation changes brightness more directly than its retardation colour: the underlying index difference and thickness remain the same. The isotropic unstressed matrix remains dark throughout rotation.
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