James Dunlop discovered NGC 6139 = D 536 on 13 May 1826 and recorded "a round nebula, about 1' diameter, bright immediately at the centre, and very faint from the bright nucleus to the margin. Another observation makes the figure rather elliptical, with a bright nucleus." He observed the cluster twice and his position is pretty poor -- 23' east of center.

John Herschel acknowledged Dunlop's discovery despite the poor position and observed the globular on 4 nights. On his first sweep of 24 Jun 1834 he logged "vB, R, pL, pgmbM, 2.5'. Evidently a globular, but moonlight very bright and near full, and I cannot see the individual stars." Two nights later he wrote "B, R, pgbM, resolvable, 2'." Another two nights later he wrote "pB, R, pgbM, resolvable, with left eye I can barely discern a few of the stars." Using the Great Melbourne Telescope on 2 Jun 1877, Joseph Turner called it "very finely stippled but too faint for any stars to be seen distinctly" (p. 137 in logbook).

200/250mm - 8" (6/27/81): faint, small, even concentration to core. Although easily visible, there was no resolution.

400/500mm - 18" (7/10/02 - Magellan Observatory, Australia): this globular appeared moderately bright and well concentrated to a bright 1' core. The halo spanned ~3.5' and the overall structure was symmetric. A number of very faint stars winked in and out of view with the seeing and the cluster appeared on the verge of extensive resolution at 171x. I didn't examine it, though, at higher power.

Notes by Steve Gottlieb