Extinction directions in a medium <i>M</i> may be found by using a polarizer <i>P</i>, a Nakamura half-shade <i>H</i>, and an analyzer <i>A</i>. These components are set up with <i>P</i> and <i>A</i> crossed and so that monochromatic light passes in turn through <i>P</i>, <i>M</i>, <i>H</i>, and <i>A</i> to enter a microscope focused on the dividing line of <i>H</i>. The extinction directions are found by rotating <i>M</i> until the field of view is of uniform brightness: they coincide then with the vibration directions of <i>P</i> and <i>A</i>. This method is considered in detail, and it is shown on theoretical grounds that, for media having a phase difference less than π/2, increased sensitivity results when a quarterwave plate <i>Q</i> is inserted, in zero azimuth with respect to the vibration direction of <i>P</i>, between <i>M</i> and <i>H</i>. The calculated increase for very small phase differences is considerable: experiment confirms this. A comparison is made with a method in which <i>H</i> is a half-shade of the Bravais type and <i>Q</i> is omitted. The present arrangement serves also as a sensitive compensator for the measurement of optical phase differences and it is, therefore, particularly suitable for measurements, such as occur in flow birefringence studies, where both extinction angles and the magnitude of the birefringence are required. This application is discussed.
H. G. JERRARD, "Determination of Extinction Directions in a Doubly-Refracting Medium of Small Phase Difference: Application to Flow Birefringence Studies," J. Opt. Soc. Am. 46, 259-262 (1956)