The jurors thus sworn hear the evidence which the trial judge permits to be offered. In determining what evidence is to be offered and considered the judge applies rules of law and the jury is authorized only to consider the evidence which is submitted to them. They have no right to take into account matters of fact not shown by the evidence, such as particular facts relating to the case which they know or believe as of their own knowledge. They are to try the case under the direction of the court as to what testimony or facts are properly admissible as affecting the verdict which they shall render.

Having heard the evidence submitted to them, the jurors are instructed by the judge as to the rules of law which they are to follow in reaching a conclusion from the evidence that is submitted, and they have no right to exercise their own judgment as to the rules and principles of law applicable to the case. The determination of the law is for the judge in the discharge of his legal duty. But the credibility of the witnesses and the weight of the evidence is for the determination of the jury in the exercise of their discretion, and the judge should not interfere with or control them in its exercise. In some states the statutes very specifically prohibit any comments by the judge as to the credibility of the witnesses or weight of the evidence; in other states and in the federal courts the judge may discuss the evidence for the enlightenment of the jury, though he cannot control the conclusions which they see fit to draw from the evidence properly presented to them ( Vicksburg, etc. R. Co. v. Putnam).