Figure 3

Figure 3: Cross-section of an infinitesimally thin bundle. The Jacobi matrix (19View Equation) relates the Jacobi fields Y1 and Y2 that span the bundle to the Sachs basis vectors E1 and E2. The shape parameters D+, D -, and x determine the outline of the cross-section; the angle y that appears in Equation (19View Equation) does not show in the outline. The picture shows the projection into the 2-space (“screen”) spanned by E 1 and E 2; note that, in general, Y 1 and Y 2 have components perpendicular to the screen.