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