139 long ago that, when an extended source of unpolarised light is viewed through a highly dichroic crystalline plate cut normal to one of the optic axes, two dark brushes-the BREWSTER'S brushes-are seen. In the interference between crossed polaroids the dark isogyres do not in general pass through the optic axial directions as they do in the case of transparent crystals. These phenomena can be broadly explained using the approximate theory mentioned in Sect. 43 where the waves propagated along any direction are taken to be linearly polarised. We have seen however that according to the...