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10. Attaching the Mechanical XY Stage
The attachable mechanical stage enables smooth specimen movement along with precise re-
positioning courtesy of the vernier scales.
Firstly remove the sprung stage clips and retain for future use. The mechanical stage then
simply screws into the tapped hole provided in the top plate of the stage.
11. Observations Between Crossed Polars
Before using crossed polars, always remove the specimen from the stage and check that
extinction is complete when the analyser is "in". Both polariser (below the sub-stage
condenser) and analyser (underneath the viewing head) are rotating. It is recommended that
the polariser is set to the reference position and then the analyser rotated so that it is at
ninety degrees to it. At this point extinction will be achieved.
12. Conoscopic Observation
A microscope is used conoscopically when the back focal plane of the objective is observed
using the Bertrand lens. In petrology birefringent specimens can produce interference
patterns in this plane that are of considerable importance. The figures can be either uniaxial
or biaxial, and can only be properly observed when objectives of high aperture are used and
the full aperture of the condenser is utilised, that is, with the upper lens in position and the
iris diaphragm opened to the full. The front lens of the condenser should be as close to the
specimen as possible. A uniaxial crystal gives a characteristic "ring and brush" whilst a biaxial
crystal gives a figure with two "eyes" or metalopes, which rotate as the specimen rotates.
The complete symmetrical figure is only obtained when the section is perpendicular to the
acute bisectrix of the optic axes of the crystal. The angle between the optic axes of a biaxial
crystal can be calculated by the method due to Mallard. Measurements between the
metalopes can be calibrated against the figure obtained from a known crystal such as quartz.
P r i o r S c i e n t i f i c
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