It is a well-known method in the measurement of birefringence to utilize a crystal's 'bevelled edge', on which appears a sequence of polarization colours, from that colour representing small relative retardation where the edge is thin, to a higher colour corresponding to the full thickness of the mineral in the microsection. In dolerites these bevelled edges on augites are extremely common and are very apparent between crossed nicols. In ordinary light, however, the majority of them are undiscernible and a grain appears to have uniform thickness over its entire area. The reason for the non-appearance of the bevelled edge in ordinary light is straightforward: the augites have excellent relief, that of the adjacent felspars is low; relief is a surface phenomena and independent of thickness, so that a thin edge of augite overlying felspar has the relief of augite, and the felspar is unnoticed.