Ray paths through a prism lens, when the incident beam is already dispersed in photon energy after passage through a composite prism. The ellipse labelled E0 indicates the position of the virtual source with size s, for which all emitted rays pass the exit slit of opening i to the right. p and q refer to the distances of the lens from the source and from the slit, respectively. The virtual sources of different photon energies (labelled E < E0 and E > E0) are displaced by Δy with respect to the in-focus source. The rays labelled A and B are those rays which cross the optical axis of the lens at the largest offset for a given photon energy.