This "Polariscope" comes with Selenite slides of two fixed and one with various amounts of wave retardation. These show birefringence and interference colors when viewed in polarized light.

Selenite slide in non-polarized light

Selenite slide

"Blue/Yellow" slide in polarized light

"Various" slide in polarized light

"Red/Green" slide in polarized light

Selenite is a common name for gypsum.

Polarizing microscopes were often equipped with Selenite stage accessories. When placed between the specimen and Analyzer, selenite acts as the "retardation plate" allowing characterization of a thin, birefringent specimen. The rainbow of interference colors resulting from the interaction of polarized light from an anisotropic specimen (e.g., crystal or geologic specimen) and selenite indicates the thickness of the specimen. A very good reference on this subject is the Olympus Microscopy Resource Center.