OPERATIONAxiolab 5 Illumination and contrast methods in transmitted light ZEISS05/2019 430037-7444-001 99• The polarization direction can be changed using the setting wheel (Fig. 4-11/4) of the analyzer.ATTENTIONThe movements of rotary knobs A and BL and the respective setting wheels are coupled witheach other. Only one control element should therefore be operated at a time and themovement of the other should not be inhibited or blocked. Otherwise, mechanical damagemay occur.NOTEIf the rotary knob BL is set to the On position, rotary knob A is automatically moved into theOn position if it is not there already.If, on the other hand, rotary knob A is set to the Off position, rotary knob BL is automaticallymoved into the Off position if it is not there already.• Place a selected crystal in the center of the crossline reticle.• Swivel in the N-Achroplan 50x/0.8 Pol objective or EC Plan-Neofluar 40x/0.9 Pol objective and focususing the focusing drive.• If necessary, close the luminous-field aperture to avoid superimposition of axial figures of neighboringcrystals on the axial figure to be examined. The smallest crystal value that can be faded out is 170 μm.• Switch on Bertrand lens BL (Fig. 4-11/1) (On position). The axial figure will appear in the field of view.• Bring the axial figure into focus using the setting wheel (Fig. 4-11/5).(4) EvaluationCrystalline anisotropic specimens can be separated into optical uni- and biaxial specimens, in each casewith an "optically positive" or "negative" character.Uniaxial crystals display a black cross when the optical axis is parallel to the direction of view.Depending on the level of birefringence and specimen thickness, concentrically arranged coloredinterference rings (so-called isochromes) may appear (see also Fig. 4-12, second row).The lines of this black cross remain closed when the stage is rotated. Depending on the section it may liewithin or outside the displayed objective pupil.With optically biaxial crystals, the cross resolves into two dark hyperbola branches (the so-calledisogyres) depending on stage rotation, which are surrounded by colored interference patternsdepending on the amount of birefringence and specimen thickness (suggestive of the figure "8").
