The board may at its discretion, issue a permit to practice optometry during the interim between examinations, to any person who has filed an application for examination which has been accepted by the board as admitting the applicant to the next examination. Such permit shall be valid only until the date of the next examination and shall not be issued sooner than thirty days following any regular examination, and no permit shall be issued to any person who has failed before the board, nor where a certificate has been revoked.