The polarised light microscope is designed to observe and photograph specimens that are visible primarily due to their optically anisotropic character. In order to accomplish this task, the microscope must be equipped with both a polariser, positioned in the light path somewhere before the specimen, and an analyser (a second polariser), placed in the optical pathway between the objective rear aperture and the observation tubes or camera port. Image contrast arises from the interaction of plane-polarised light with a birefringent (or doubly-refracting) specimen to produce two individual wave components that are each polarised in mutually perpendicular planes.