Light from a bright point source with condenser and iris passes through a yellow filter and two crossed polaroids onto a distant screen. The half-wave plate, consisting of two identical quarter-wave plates, is then positioned between the two crossed polaroids at the angle which produces the greatest illumination on the screen. Remove the plate and cross the polaroids. The half-wave plate is then inserted set at the angle for darkness. The plate is rotated somewhat less than 45 degrees, producing some light. The analyzer must be rotated by twice the angle of the plate rotation to again produce darkness.
The quarter wave plates have thickness chosen for yellow light.