Heinrich d'Arrest discovered NGC 3991, along with NGC 3995, on 5 Feb 1864 with the 11" refractor at the Copenhagen Observatory. His position (measured on 3 nights) is accurate.

400/500mm - 17.5" (2/24/90): moderately bright, fairly small, edge-on SSW-NNE, 1.0'x0.3'. This object has a bright stellar knot at the NNE end (about 25" from the center) giving an unusual asymmetric appearance! First of three striking elongated systems with NGC 3994 3.7' SSE and NGC 3995 3.9' SE.

600/800mm - 24" (5/25/14): moderately to fairly bright, very elongated 4:1 SSW-NNE, 1.0'x0.25'. At 375x, this irregular galaxy has a striking, asymmetric appearance with a very bright, elongated knot (site of very active star formation), 15"x8", at the NNE end. At 375x the knot is slightly skewed to the major axis of the longer (but fainter) portion of the galaxy. There is no central brightening, in fact, the galaxy dims just south-southwest of the knot (though not completely detaching the knot) and then brightens along the southern half. At best, there was a very small, inconspicuous core. First in an excellent trio (KTG 39) with NGC 3994 3.8' SSE and the remarkable NGC 3995 3.8' SE.

Notes by Steve Gottlieb