This maximum colour is often diagnostic of an anistropic mineral and it is observed in sections that display simultaneously the maximum and minimum refractive indices. The numerical difference between the two indices is the birefringence. For example, augite has a maximum and minimum refractive indices of 1.724 and 1.700 giving a birefringence of 0.024. The Michel Levy chart summarizes the relationships between interference colours, birefringence and thin section thickness.