Summary: The accurate determination of α: and 2V in members of the aegirine-augite series often presents difficulties owing to strong absorption and dispersion, especially as high angles of tilt on the universal stage are often involved. Both of these angles depend on the location of α, and this operation is the one most subject to error.
An alternative procedure is the determination of A:, the angle between an optic axis and . This angle is very easily and accurately determined and varies systematically with composition. It can be derived from the values for α: and 2V, while conversely values of α: and 2V can be obtained from A: by reference to standard curves.