After his incapacity has been judicially determined, a person of unsound mind can make no conveyance or other contract, nor delegate any power or waive any right until his restoration to capacity. But a certificate from the medical superintendent or resident physician of the insane asylum to which such person may have been committed, showing that such person had been discharged therefrom cured and restored to reason, shall establish the presumption of legal capacity in such person from the time of such discharge.