William Herschel discovered NGC 6638 = H I-51 = h3748 on 12 Jul 1784 (sweep 236) and recorded "a pretty cons. easily resolvable R nebula, vBM and faint red perceivable. A second miniature of that near the 42nd Comae [M53]." JH made the single observation "B; S; R; psbM; diam in RA = 4.5s; barely resolved; a very delicate object; doubtless a globular cluster." His position is accurate.

400/500mm - 17.5" (5/10/91): fairly bright, fairly small, round, 2.5' diameter, gradually increases to a bright core. The halo has a fairly even surface brightness. A few extremely faint mag 15-16 stars are resolved at the mottled edges. Located 3.6' N of mag 9.4 SAO 186904.

Notes by Steve Gottlieb