sharing our knowledge.
Photographic gallery.  Thousands of particles under the microscope.
Uniaxial Interference Figure

Uniaxial Interference Figure, Sodium Nitrate Fusion Sample

This is a conoscopic view of a crystal of sodium nitrate from a fussion mount. It shows a centered view down the optic axis. The melatope at the center, the isogyres (Maltese cross), and the isochromes are all visible.

Transmitted Cross Polarized Light, Conoscopic View


Sodium nitrate freezes in large crystals with a variety of orientations when cooled under a coverslip. The crystal that shows the least change when rotated between crossed polarizing Filters is the closest to 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 a Maltese Cross. The black area at the center of the cross is the projection of the optic axis at the center of the isochromes. The arms of the cross are the locations where the omega and epsilon prime orientations align with the two polarizing filters.

Significance in the Environment:

The uniaxial interference figure identifies the particle as belonging to the uniaxial group of crystals, tetragonal or hexagonal. By using a compensator plate the optical sign of the crystal can be determined. Seeing any part of the uniaxial interference figure identifies the orientation of the omega refractive index. The omega refractive index is always normal to the isogyres and tangential to the isochromes at the center of the field of view. If the uniaxial interference figure is centered in the field of view then only the omega refractive index can be measured in that orientation. If uniaxial interference figure is not centered then the omega and some epsilon prime refractive indices can be measured. If only a "flash figure" is seen then both the omega and the true epsilon refractive indices can be measured.

Characteristic Features:

The uniaxial interference figure contains the melatope (projection of the optical axis), the isogyres (dark bands), and the isochromes. The isochromes are always circles of colored bands centered on the melatope. With circular polarized light the isogyres disappear. The isogyres always travel up and down and from right to left or left to right across the field of view except for the "flash figure". The flash figure occures when the epsilon refractive index is in the plane of the stage. The flash figure disappears typically within a rotation of five degrees or so.