To Determine the Vibration Direction in a Mineral.

  1. Rotate the grain on the stage to extinction. In this position the vibrations directions of the grain are parallel to the crosshairs of the microscope which are themselves parallel to the polarization directions of the microscope.
  2. Rotate the stage 45°, clockwise. The vibration direction that was parallel to the NS crosshair is now aligned NE-SW. The grain should be brightly illuminated at this point. Note the interference colour exhibited by the grain and locate this colour on Plate 1 and record its retardation.
  3. Insert the Gypsum Plate. The slow ray vibration direction of the plate is aligned NE-SW. Is the interference colour now exhibited by the grain higher or lower than the recorded in step 2, i.e. has the colour moved up (to the right) or down (to the left) by 550 nm.
  4. If the colour increased, went up the chart, then the slow ray in the accessory plate is parallel to the slow ray in the mineral grain. If the colour decreased, went down the chart, then the slow ray of the accessory plate is parallel with the fast ray of the grain.