NMAC BTWN DSNDING B747 AND BE20 VFR AT 17500 FT. BE20 HAD THE B747 IN SIGHT. CTLR HAD ISSUED THE BE20 TFC TO THE B747 AND A DSCNT A PLT'S DISCRETION. NO RPT OF TCASII ALERTS OR EVASIVE ACTION BY EITHER ACFT.

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ACFT #1 WAS INBOUND TO BOSTON ON A SW HDG. WE CHKED IN DSNDING TO FL240. ACFT #2 WAS NEBOUND VFR AT 17500 FT. I DSNDED ACFT #1 TO FL180 REF ACFT #2. WHEN THE ACFT WERE ABOUT 15 MI APART; I ISSUED TFC TO ACFT #1 TELLING HIM HE HAD TFC 11-12 O'CLOCK AND 15 MI NEBOUND VFR AT 17500 FT; AND THEN I ISSUED A PLT'S DISCRETION DSCNT TO CROSS SCUPP INTXN AT 11000 FT. AT 10 MI I ISSUED TFC TO BOTH ACFT. ACFT #1 SAID HE WAS LOOKING AND ACFT #2 SAID HE HAD THE B747 IN SIGHT. I INFORMED ACFT #1 THE BE20 HAD HIM IN SIGHT; AND HE ACKNOWLEDGED. A FEW MOMENTS LATER; ACFT #2 MADE A XMISSION I DIDN'T HEAR AND I ASKED HIM TO SAY AGAIN. I THOUGHT HE SAID 'WE SEE THE B747 BELOW US.' I THANKED HIM FOR THE RPT. (AFTER REVIEWING THE TAPES; I FOUND OUT HE WAS ASKING THE ALT OF THE B747 BELOW HIM.) AT THIS TIME I NOTED THE ALTS IN THE DATA BLOCKS. ACFT #1 WAS AT 17400 FT DSNDING; AND ACFT #2 WAS STILL LEVEL AT 17500 FT. THE PLT OF THE B747 THEN ASKED ME WHY I HAD ISSUED A DSCNT CLRNC TO HIM WHEN HE HAD TFC. I TOLD HIM I HAD ISSUED A PLT'S DISCRETION DSCNT AND HAD EXPECTED HIM TO DSND IN REF TO THE TFC OR WAIT UNTIL I HAD INFORMED HIM THAT HE WAS CLR OF THE BE20. HE SAID THAT SINCE HE WAS UNDER RADAR CTL IT WAS MY RESPONSIBILITY TO PROVIDE SEPARATION AND THAT HE HAD ALMOST HIT THE OTHER ACFT. IN RETROSPECT I PERHAPS SHOULD HAVE MADE IT CLR TO THE PLT OF ACFT #1 THAT I WAS EXPECTING HIM TO WAIT UNTIL HE HAD VISUAL CONTACT WITH THE VFR ACFT. I COULD ALSO HAVE SIMPLY DELAYED HIS DSCNT UNTIL I OBSERVED THE RADAR TARGETS PASSING; BUT I WAS GETTING BUSY AND DIDN'T WANT TO FORGET HIM. IN ANY EVENT CLR COM OF MY INTENT AND BETTER UNDERSTANDING OF PLT'S DISCRETION FOR HIM WOULD HAVE AVERTED THE PROB.