Annu. Rev. Astron. Astrophys. 1992. 30:
311-358 Copyright © 1992 by . All
rights reserved |

*time delay* associated with the image at ^{(i)}, relative
to the direct ray in the absence of the lens, is given by
(Refsdal 1964b,
Cooke &
Kantowski 1975,
Kayser & Refsdal
1983,
Borgeest 1983,
Schneider 1985)

The quantity *t ^{(i)}* itself cannot be measured, but the

*t*() is taken to be
the fundamental quantity and the lens
Equation 2 is obtained by invoking Fermat's principle, which
states that images are formed at stationary points of *t*
(Weyl 1922,
Schneider 1985,
Blandford &
Narayan 1986).
Equation 11 can be generalized to multiple lenses
(Blandford &
Narayan 1986,
Kovner 1987b),
as well as to nonstationary gravitational fields
(Kovner 1990,
Nityananda &
Samuel 1992).