Sketch of two light rays through four consecutive planes, with 0 ≤ λ_q <  λ_r <  λ_s <  λ_t. The rays are not deflected in the lens planes. The first ray has its vertex at λ_q and encloses an angle θ with the fiducial ray; the second ray with vertex at λ_r encloses an angle ϑ with the fiducial ray. Both rays intersect at λ_s. The geometry of this figure yields the relation (25) between distance matrices.