  The recent discovery of light moving backwards in time, when it propagates in
a suitable dispersive medium, obliges us to reexamine the Kramers-Kronig
relations. In their usual form, they are dealing with usual light (moving
forward in time), and they are a consequence of causality. In a similar
fashion, we derive the appropriate form of these relations for light moving
backwards in time, starting, instead of causality, from a principle of
anti-causality, appropriate to such a light.
