No person shall be convicted of perjury if he retracted his false statement in the course of the same proceeding in which it was made before it became manifest that the falsification was or would be exposed. Statements made in separate hearings at separate stages of the same trial or administrative proceeding shall be deemed to have been made in the course of the same proceeding. The burden of injecting the issue of retraction is on the defendant, but this does not shift the burden of proof.