(17)No commander or lieutenant-commander or lieutenant shall be required to sit as a member of a court-martial when four officers of higher rank and junior to the president can be assembled at the place where the court-martial is to be held, but the regularity or validity of any court-martial or of the proceedings thereof shall not be affected by any commander, lieutenant-commander or lieutenant being required to sit or sitting thereon under any circumstances and when any commander, lieutenant-commander or lieutenant sits on any court- martial, the members of it shall not exceed five.
