The angular intervals for the DSW phase and group velocities (Δθ ph and Δθ gr) are different. The phase velocity interval is proportional to η and even for the most strongly birefringent natural crystals is very narrow Δθ ph ≈ 1° (rutile) and Δθ ph ≈ 4° (calomel). However the physically more important group velocity interval is substantially larger (proportional to η). Calculations give Δθ gr ≈ 7° for rutile, and Δθ gr ≈ 20° for calomel.