When the Sénarmont compensator is used for measuring birefringence in double refraction of flow instruments, the cross of isocline is seen to undergo a scissors-like collapsing motion as the analyzer is rotated. The position and intensity of the collapsing cross have been calculated, and are in agreement with experimental observations. From this analysis, we can directly demonstrate the end-point sensitivity of the compensator.
ALEXANDER RICH, "Use of the Sénarmont Compensator for Measuring Double Refraction of Flow," J. Opt. Soc. Am. 45, 393-394 (1955)