The statute which provides that “the courts shall not charge juries with respect to matters of fact,” prohibits the expression by the judge in his charge to the jury of his opinion as to the credibility of witnesses. Gen. Sts. e. 115, § 5. Commonwealth v. Barry, 9 Allen, 276. In the case at bar a witness for the government testified that he bought liquor of the defendant for the purpose of testifying against him. The presiding judge instructed the jury that “ if the witness, intending honestly to aid the government in breaking up a traffic forbidden by law, visited the place where the law was supposed to be violated, for the purpose of obtaining evidence, it would not detract from the weight of his testimony in the opinion of the court.” We are of opinion that this instruction was erroneous. Whether the circumstances under which the witness obtained evidence, and the motives under which he acted, would or would not detract from the weight of his testimony, was a question exclusively for the jury. Although an instruction was added that “ the whole matter was before the jury to give such weight to the testimony as they saw fit,” yet the effect of the whole instructions was to throw the weight of the judge’s opinion in the scales against tne defendant. This was a violation of the rule prescribed by the statute. Exceptions sustained.