The county commissioners, or, in Boston, the penal institutions commissioner, subject to the approval of a justice of the court which imposed the sentence, after six months from the time of sentence, may discharge a person sentenced to the house of correction, upon a conviction under the provisions of section sixty-two of chapter two hundred and seventy-two of being a common nightwalker, if they are satisfied that the prisoner has reformed.
Mass. Gen. Laws ch. 127, § 143