Refraction index measurement This page illustrates different microscopic techniques for the estimation of the refractive index of small crystals. The pictures throughout the page show how quartz can be seen in a series of immersion liquids with refraction index around  its ordinary index by using the phase contrast microscopy, the oblique illumination, the Becke line method and the dark field. The use of a monochromator to illuminate the stage of the microscope allows more quantitative determination. The images below also show the appearance of the crystals in monochromatic light with wavelengths around the crossing point between dispersion curves of the solid and the liquid.