2. When the informer is entitled to the penalty or part of the penalty, upon the conviction of an offender, he is or is not a competent witness, accordingly as the statute creating the penalty has or has not made him so. 1 Phil. Ev. 97; Rosc. Cr. Ev. 107; 5 Mass. R. 57; 1 Dall. 68; 1 Saund. 262, c. Vide articles Prosecutor; Rewards.