Uniaxial Interference Figure, Sodium Nitrate Fusion Sample
Transmitted Circular Polarized Light, Conoscopic View

Sodium nitrate freezes in large crystals with a variety of orientations when cooled under a coverslip.
A crystal that shows the lowest interference colors when viewed with circular polarized light is near
an optical axis normal position. When viewed conoscopically by using the Bertrand lens, phase telescope,
pin hole eyepiece, or just viewing down the optical tube with the eyepiece removed shows the projection
of the optic axis at the center of the isochromes.