Determination of the Extinction Angle of a mineral

All anisotropic minerals have what is known as an extinction position. These positions are identified as dark positions (with no colour) as the microscope stage is rotated. For each rotation there are 4 such extiction positions (90 degrees apart).
The Extinction Angle is defined as the angle between one of the cross hairs and a line of cleavage in the mineral at the extinction position.
To determine this angle, The angle reading at the extinction position is read from the microscope stage, and then the microscope stage is rotated until the cleavage lines become parallel to one of the cross hairs. The reading at his point is also taken. The difference between the two readings will give the extinction angle.

extincton position - reading1

parallel cleavage position - reading2

Extinction angle  =  reading 1 - reading 2

If the extinction position occurs when the cleavages are parallel to the cross hair, then it has an extinction angle of zero. Such minerals are said to have a  "Parallel" extinction. eg. Orthopyroxene