Portions of the funds held by the Director or by any state correctional facility, which belong to prisoners may, in the discretion of the Director, be invested in bonds of the Commonwealth of Virginia or of the United States or in federally-insured investments. In determining how to invest the funds, the Director shall balance any long-term investments with those which permit ready accessibility to the funds. Any income or increment of increase received from the bonds or investments may be used by the Director for the benefit of the prisoners under his care.