Section 56. Except as otherwise provided, an officer of the commonwealth whose salary is fixed by law, or any employee of the commonwealth receiving regular compensation therefrom shall not be entitled to a witness fee before any court in a cause in which the commonwealth is a party. An officer whose compensation is derived solely from fees shall not be entitled to receive more than one fee as a witness for a day's attendance on court under one or more summonses in behalf of the commonwealth, and the said fee shall be apportioned by the clerk among the cases in which he is summoned. Nothing in this section shall be construed to authorize the payment of a witness fee to any member of the police department of said commission if he is on duty at the time of his attendance on court as a witness.