85
Biaxially positive crystals:
The interference fringes move from the convex
to the concave side of the isogyres when the
compensator is operated.
Biaxially negative crystals:
The interference fringes move from the concave
to the convex side.
Possible errors
Polarizers damaged (discoloured) by powerful
light sources or dirty.
Objectives or condenser strained through
mechanical damage.
Beamsplitter or filter between the polarizers.
Mounting medium for transmitted light
specimens is birefringent.
Further sources of error on page 72.
Fig. 59a
Determination of the optical character of uniaxial structures
Left: Positively uniaxial crystal, cut vertically to the optical axis
Right: Negatively uniaxial crystal, cut vertically to the optical axis
1
Diagram of the vibration directions in the object and in the compensator
2
Change in the interference pattern when a quarter-wave compensator is used
3
Change in the interference pattern when a whole-wave compensator is used
Fig. 59b
Chart for determination of the optical character
