ACR ON DSCNT HAS NMAC WITH SMA.

Narrative

OUR FLT WAS PROCEEDING UNEVENTFULLY FROM RDG TO LNS. WE WERE DSNDING FROM 4000 FT FOR A VISUAL APCH; AND WERE UNDER THE CTL OF READING DEP. THE CONFLICTING TFC WE WERE ABOUT TO ENCOUNTER HAD NOT BEEN RPTED TO US BY ATC. THE FO WAS THE PF. I WAS LOOKING FOR OTHER ACFT. WHILE I WAS READING THE DSCNT AND APCH CHKLISTS; I NOTICED MOVEMENT OUT OF THE R 'CORNER' OF MY EYES. I COULD IMMEDIATELY TELL WE WERE NOT GOING TO HIT THE OTHER ACFT; BUT IT WAS CLOSE. WE DID NOT TAKE ANY EVASIVE ACTION BECAUSE WE HAD NEITHER THE TIME NOR THE NEED TO DO SO. THERE WAS VERY LITTLE TIME TO IDENT THE OTHER ACFT; EXCEPT WE COULD TELL THAT IT WAS A SINGLE ENG LOW FIXED WING. WE WERE CONVERGING ON AN APPROX 45 DEG ANGLE. WE WERE DSNDING FROM ABOVE THE OTHER ACFT. IT IS POSSIBLE THAT WE SAW THE OTHER ACFT ONLY AT THE LAST SECOND BECAUSE HE WAS BELOW OUR NORMAL VIEWING CAPABILITIES OUT OF THE COCKPIT. IT MIGHT HAVE BEEN BELOW THE WINDOWS. AFTER THE ENCOUNTER I ASKED READING DEP IF THEY SAW ANY OTHER ACFT ON THEIR RADAR; AND THE CTLR SAID NO. THE OTHER ACFT DID NOT SHOW UP ON THE RADAR SCOPE AT READING; APPARENTLY. I INTEND TO BE EVEN MORE VIGILANT IN THE FUTURE. I DON'T KNOW WHY THE OTHER ACFT DID NOT SHOW UP ON READING'S RADAR. I AM AWARE OF THE REQUIREMENTS OF THE SEE-AND-AVOID CONCEPT.