30. In any such appeal, the appellant shall not have the right to adduce further evidence than that already given in the first instance, unless it be shown, to the satisfaction of the board, that the existence of such further evidence has come to his knowledge since the first hearing of the case, or unless the board, at the time of hearing such claimant, deems it right to allow such further evidence.
R. S. 1964, c. 138, s. 30.