FLC OF MD80 TAKES EVASIVE ACTION WHEN TCASII INDICATES TA THEN RA WHEN ISSUED A CLB AND MAINTAIN VISUAL SEPARATION.

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DEPARTING ATL NBOUND; WE WERE TOLD TO LEVEL OFF AT APPROX 15000 FT. AUTOPLT LEVELED OFF AS DIRECTED. ZTL ADVISED US THAT THERE WAS SW4 TFC AT 1 O'CLOCK AND APPROX 5 MI AT 16000 FT AND TO EXPECT FURTHER CLB PASSING THAT TFC. BOTH CAPT AND I RPTED TFC IN SIGHT AND CTR TOLD US TO MAINTAIN VISUAL ACFT WITH THE SW4 AND CLB AND MAINTAIN A HIGHER ALT. CAPT AND I BELIEVED SW4 WOULD PASS TO THE E OF OUR ACFT; SO AN APPROX 1000-1500 FPM CLB WAS INITIATED. TCASII SOON GAVE 'TFC; TFC' FOLLOWED BY 'MONITOR VERT SPD.' TCASII SHOWED GREEN BAND TO APPROX 300 FT AND RED BAND IN EXCESS OF 300 FT. CAPT STARTED TO LEVEL OFF ACFT WITH AUTOPLT WHILE STILL MAINTAINING VISUAL CONTACT WITH OTHER ACFT. BY THE TIME THAT WE HAD ACTUALLY LEVELED OFF; THE SW4 FLEW DIRECTLY OVERHEAD IN WHAT I BELIEVE WAS NO FURTHER VERTLY THAN 300 FT. AS WITH ANY ACFT INCIDENT OR ACCIDENT; THERE WERE A NUMBER OF FACTORS INVOLVED IN THIS TFC CONFLICT; ALL OUR FAULT. FIRST OF ALL; I BELIEVE THAT GIVING CLRNCS THAT DIRECT AN ACFT TO 'MAINTAIN VISUAL CONTACT AND CLB TO...' A CERTAIN ALT SHOULD BE AVOIDED DURING NIGHTTIME CONDITIONS. EVEN THOUGH I SAW THE ACFT; I THINK THAT THE NIGHTTIME VISUAL CONDITIONS MADE THE DISTANCE THAT WE WERE FROM THE ACFT VERY DECEPTIVE. IT APPEARED THAT THE SW4 WAS MUCH FURTHER VISUALLY FROM US THAN WAS ACTUALLY THE CASE. TO AVOID THIS IN THE FUTURE; I WILL ENSURE THAT NO CLB IS STARTED UNTIL THE OTHER ACFT HAS VISUALLY; AND USING TCASII; PASSED US. DISTANCE AND DIRECTION OF FLT FROM OTHER ACFT ARE DIFFICULT TO DETERMINE AT NIGHT. ANOTHER FACTOR IN THIS CONFLICT WAS THE SLOW RATE AT WHICH WE STOPPED OUR APPROX 1000-1500 FPM CLB. THE ACFT WAS BEING FLOWN BY THE CAPT WHO AT THE TIME WAS USING THE AUTOPLT. THE AUTOPLT HAS BEEN MANUFACTURED TO CLB AND DSND AS SMOOTHLY AS POSSIBLE. THIS CAN RESULT IN SLOW CHANGES OF DIRECTION; AND IT IS IN TIMES LIKE THE ABOVE SCENARIO THAT THE AUTOPLT MUST BE TURNED OFF AND EXTREME CORRECTIONS TAKEN TO AVOID HITTING ANOTHER ACFT. THE LAST FACTOR IS TO BELIEVE AND TRUST YOUR TCASII. DO NOT TRUST YOUR OWN VISUAL CUES. I BELIEVE THAT HAD WE RAPIDLY FOLLOWED THE TCASII RA; WE WOULD HAVE HAD 1000 FT VERTLY BTWN ACFT. I HOPE THAT THIS WILL ALSO ALLOW US TO ALL THINK ABOUT ACCEPTING NIGHTTIME VISUAL CLB CLRNCS. IT IS SOMETHING THAT I DO NOT PLAN TO DO IN THE FUTURE. IT IS BECAUSE OF TCASII THAT I AM ABLE TO SUBMIT THIS NASA ASRS FORM.