If, in a diaphanous homogeneous medium with a given refractive index, areas with different refractive indices develop, a plane between these areas will become visible provided the difference between the local refractive indices suffices for reflection of light. Conversely, where such planes are observed, a difference between the refractive indices on either side may be assumed. Finally, change of the localization of the planes indicates alteration of the refractive indices. The above approach is applicable to slitlamp images of the lens.