Whenever the Chief Justice and the Attorney General shall jointly represent to the President that it is expedient, for the purposes of expediting the trial and determination of any criminal or civil causes or matters pending in the High Court, that one or more suitably qualified persons should be appointed in addition to the judges of the High Court to exercise either or both the criminal or civil jurisdiction of that court, the President may, from time to time, by instrument under the public seal of Kenya, appoint such a person or persons to be a commissioner or commissioners of assize to hold office, subject to the terms and conditions of such instrument, for such period, or for such criminal or civil session or part of a criminal or civil session of the High Court, as shall be specified in the instrument.