By properly combining two prisms made of different materials, it is possible to
In case of prism combined in a way that refractive angles are to be reversed with respect to the each other. Then, the net deviation of the yellow ray can be determined as,
δy=(μy-1) A-(μy’-1) A’
And, the net angular dispersion must be
δy-δr=(μy-1) A
Hence, by choosing those conditions, we can have the above mentioned cases.