The Babinet-Soleil Compensator is a variable waveplate which, for example, can convert circularly polarised light into linearly polarised light or vice versa. It comprises two opposed birefringent crystal wedges with a compensating crystal block in optical contact with the smaller wedge. Both wedges are cut with the optic axis parallel to their long edges, and the compensating block has its axis at right angles. In operation, the large wedge is translated across the smaller, thus presenting a variable path length difference to an optical beam passing through the instrument. The compensating block ensures that this difference is uniform across the aperture.