§  51-b.  Mirrors  in  connection  with self-service elevators. In all  multiple dwellings in which there are one or more self-service passenger  elevators, there shall, pursuant to such regulations as  the  department  shall  prescribe,  be  affixed  and  maintained  in each such elevator a  mirror which will enable persons prior to entering into such elevator to  view the inside thereof to determine  whether  any  person  is  in  such  elevator.