To determine whether an anisotropic mineral is uniaxial or biaxial, and whether it is optically positive or negative, requires obtaining an interference figure. Measuring 2V, for biaxial minerals, also requires an interference figure. We obtain these figures using a conoscopic lens below the microscope stage, and a Bertrand lens above the upper polarizer, when looking at a grain with XP light. Figure 5.23 shows this arrangement.
Any uniaxial or biaxial mineral (whether in a grain mount or a thin section) will, in principle, produce a visible interference figure; isotropic minerals will not. We must take care to choose grains without cracks or other flaws so light can pass through without disruption. In addition, for some purposes, we need grains with a specific orientation.
Having chosen an appropriate grain, obtaining an interference figure is relatively straightforward. We focus the microscope using PP light and high magnification. (If perfect focus is ambiguous, it often helps to focus first at low magnification.) Then we insert the upper polarizer to get XP light. If the microscope is properly aligned, the grain will still be in focus. Next, we fully open the substage diaphragm, insert the substage conoscopic lens (if it is not already in place), and then insert the Bertrand lens above the upper polarizer.
As seen in Figure 5.64, the conoscopic lens focuses polarized light so it enters a mineral grain from many different angles simultaneously and focuses on a small area. Double refraction occurs for all rays and different wavelengths have different retardations. At the upper polarizer, slow rays and fast rays combine to produce interference colors. And above that polarizer, the Bertrand lens focuses the light so it is parallel again. The Bertrand lens also magnifies the image.
So, the two special lenses together permit us to see an interference figure and examine light traveling through a crystal by many paths in a single view. Without the lenses, we would have to look at many different crystals to obtain the same information. (Some older microscopes do not have Bertrand lenses, but we can still see an interference figure by removing an ocular and inserting a peep sight, or by just peering down the tube. The figures, however, will be quite small.)