Sec. 16. LEAVING EMPLOYMENT BEFORE BECOMING ELIGIBLE. When any member of such Pension System shall leave the employment of such Police Department except as specifically provided for herein, either voluntarily or involuntarily, before becoming eligible for a retirement or disability pension, he shall cease to be a member of such Pension System. When a member has left the service of the city Police Department as aforesaid and has therefore ceased to be a member of such Pension System, if such person shall thereafter be re-employed by the city Police Department he shall thereupon be reinstated as a member of such Pension System provided he is in good physical and mental condition as evidenced by a written certificate executed under oath by a duly licensed and practicing physician or physicians in the city, satisfactory to the Pension Board. Prior service of such member with such city Police Department shall not be counted toward his retirement pension unless such member returns to the service within two (2) years from his separation therefrom, and also shall, within six (6) months after his re-employment by the city in the Police Department, make a written application to the Pension Board for reinstatement in the Pension System.