Section 3. The by-laws of an optometric service corporation may contain any lawful provisions approved by the commissioner and shall provide that a majority of the directors shall at all times be persons approved in writing by an optometric society incorporated in the commonwealth not less than five years and having not less than four hundred registered optometrists as members, and that not less than one third of the directors shall be persons who are or who agree to become subscribers to the non-profit optometric service plan. The by-laws of such a corporation may define the qualifications of those persons eligible to become subscribers as provided in section five. Any such corporation may adopt such rules and regulations as may be consistent with the provisions of this chapter. Any optometric service corporation may contract with any agency of the United States of America, of the commonwealth or of any city or town within the commonwealth for the purpose of providing optometric service.