ABSTRACT
Consider a plane parallel birefringent plate illuminated by linearly polarized light. The incident vibration may be broken down into two coherent vibrations directed along the neutral lines of the plate. A point-source of white light illuminates a birefringent prism, which produces lateral separation, and the emerging beam falls on a screen. A Babinet compensator consists of two quartz wedges of equal angle. The path difference given by the cellophane sheet is determined by the distance between the two dark fringes belonging to the fixed and the displaced fringe system. Between two polarizers, localised in the device, a system of concentric rings are observed centred at the point where the thicknesses of the two constituent elements are equal. If the uniaxial crystal is cut perpendicular to the optical axis, one observes, in white light, coloured rings centered on the trace of the optical axis and a dark or a white cross.
