B727 FLT CREW RECEIVES A 'LOW ALT ALERT' FROM THE CTLR DURING APCH TO BOS.

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NIGHT VMC; 10+ MI VISIBILITY. APCHING BOS AT APPROX XA00 FROM THE S FOR VECTOR TO RWY 27. APCH WAS VECTORING US TO FOLLOW A DC9 APCHING FROM THE N. WE WERE VECTORED AT 4000 FT OR 5000 FT MSL ONTO A WIDE DOWNWIND (2+ MI) FOR L BASE TO RWY 27. WE HAD ARPT AND RWY AND TFC IN SIGHT (AT 4000 FT MSL) AND CALLED BOTH TO APCH CTL. WE WERE CLRED THE VISUAL TO RWY 27 BEHIND THE DC9. THE CAPT STARTED A DSCNT AND CONTINUED THE DOWNWIND TO ALMOST ABEAM THE TFC BEFORE TURNING BASE. I WAS WATCHING THE TFC FOR SPACING AND LOST SIGHT OF THE RWY AS WE GOT LOWER. WHEN I KNEW THE TFC WAS NO FACTOR; I STARTED LOOKING FOR THE RWY AND COULDN'T FIND IT. I COULDN'T SEE THE ROTATING BEACON EITHER (IT SHOULD BE HIGHER AND/OR AWAY FROM THE TERMINAL AREA!). AROUND 2500 FT MSL; I WAS TRYING TO CORRELATE THE ACFT POS/PATH; INSTS (LOC/HDG) AND CAPT'S GAZE TO WHERE HE THOUGHT THE RWY WAS. TO FIND THE RWY. AT ABOUT 1500 FT MSL; I COULD TELL THE CAPT DIDN'T HAVE THE RWY IN SIGHT EITHER. AT 1400 FT MSL; THE SO SPOKE UP TO SAY THE ARPT WAS FURTHER R THAN WE WERE AIMING AND POINTED OUT THE LNDG TFC. AT 1300 FT MSL; TWR ISSUED A LOW ALT ALERT CAPT IMMEDIATELY LEVELED OFF; REACQUIRED THE ARPT/RWY; THANKS TO THE SO; AND WE PROCEEDED FOR AN UNEVENTFUL LNDG. I SHOULD HAVE SPOKE UP SOONER AND HIGHER ABOUT NOT HAVING THE RWY IN SIGHT. NEED TO RAISE MY PERSONAL MINIMUMS FOR BEING 'IN THE SLOT' FROM 1500 FT AGL TO 2000 FT AGL; TO INCREASE REACTION TIME. 'TRUST; BUT VERIFY' STILL SOUND ADVICE.