In this paper the influence of the linear heterogeneity on the ray path in some reflecting prisms is considered. It is stated that all the prisms discussed here have some focusing properties. The focal length and focus coordinates are calculated as the functions of magnitude and direction of refractive index gradient.
Barbara Lisowska, Zygmunt Bodnar, and Florian Ratajczyk, "The Influence of Linear Glass Heterogeneity on the Optical Beam Pass through Certain Reflecting Prisms," Appl. Opt. 7, 2270-2279 (1968)