No licensed optometrist shall practice his profession as an employee of any unlicensed person, firm or corporation, provided that said prohibition shall not apply to health service organizations, hospitals, other optometrists or ophthalmologists. Nothing herein contained shall prohibit any registered optometrist or optometrists from continuing his employment in the mercantile establishment in which he or they were employed on June 28, 1963. No rule of the board shall prohibit the practice of optometry on a lessee or sublessee basis in or on the premises of a retail, commercial or mercantile establishment.