Historical Identification: Per Dreyer, NGC 3950 (= GC 5591, 4th Lord Rosse, 1860 RA 11 46 22, NPD 41 19.4) is "extremely faint, 2.6 arcmin north of h 1009", "(JH) 1009" being NGC 3949. The position precesses to RA 11 53 41.4, Dec +47 53 53, only 0.8 arcmin north of the galaxy listed above, the description fits (the galaxy is only 1.6 arcmin north of NGC 3949, Lord Rosse's position and distance estimate being roughly an arcmin larger than the correct values) and there is nothing nearby that could possibly fit the description, so the identification is certain.
Physical Information: Based on a recessional velocity of 22365 km/sec, a straightforward calculation suggests that NGC 3950 is about 1.04 billion light years away. However, for objects at such a distance, we need to take the Universal expansion into account. Doing that shows that the galaxy was about 955 million light years away when the light by which we see it left the galaxy, about 990 million years ago (the difference between the two numbers being due to the expansion of the intervening space in that interval). Given that and its apparent size of 0.40 by 0.35 arcmin, the galaxy is about 110 thousand light years across.

Historical Identification: Per Dreyer, NGC 3951 (= GC 2605 = JH 1010 = WH III 342, 1860 RA 11 46 22, NPD 65 49.5) is "very faint, considerably small, very little extended".
Physical Information:

Historical Identification: Per Dreyer, NGC 3952 (= GC 2607 = JH 1012 = WH III 612, 1860 RA 11 46 28, NPD 93 13.4) is "considerably faint, considerably small, a little extended 90°±, brighter middle, mottled but not resolved".
Physical Information:

Historical Identification: Per Dreyer, NGC 3953 (= GC 2606 = JH 1011 = WH V 45, 1860 RA 11 46 29, NPD 36 53.1) is "considerably bright, large, extended 0°±, very suddenly brighter middle and large mottled but not resolved nucleus". The position precesses to RA 11 53 49.8, Dec +52 20 11, on the northern rim of the nucleus of the galaxy listed above and well within its outline, the description fits and there is nothing else nearby, so the identification is certain.
Discovery Notes: Until October 2006 it was thought that Méchain's observation was of what became NGC 3992 (which see for a detailed discussion of Méchain's observation), but it was then shown by Henk Bril that what Méchain actually observed was NGC 3953; but neither the Herschels nor Dreyer were aware of the prior observation of either object, causing the lack of any mention of Méchain in either NGC entry.
 Messier's final catalog did not include either of the "nebulae" which became NGC 3953 or NGC 3992, but in response to a letter sent from Méchain to Messier, describing three objects observed by Méchain, Messier examined the regions involved, adding one of Méchain's objects to his catalog (as M97), but not bothering to publish the others. However, one of them was added to Messier's catalog by Owen Gingerich in 1953 (as M108), and Messier must have observed the other (which became NGC 3953), as in his personal copy of his final catalog Messier wrote a description of the object, using the right ascension of NGC 3953 and the declination of NGC 3992 (also added to Messier's catalog by Owen Gingerich in 1953 as M109). This unfortunate concatenation of separate positions clouds the issue, but makes it almost certain that Messier observed both objects, as indicated in the discovery credits for both entries (both of which are on this page).
Designation Note: Bril asserts that since Méchain had been thought to be the original discoverer of M109 (= NGC 3992), and what he actually observed was not that object but NGC 3953, that galaxy should be called M109 (hence the need to deny that possibility in the title of this entry). However, the reason that Gingerich added M108 and M109 to the Messier Catalog was that Messier observed them, and whether or not Méchain also observed them was irrelevant. All that Bril's discovery does is to remove Méchain from the list of observers of M109, and add him to the list of observers of NGC 3953.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 1240 km/sec (and H₀ = 70 km/sec/Mpc), NGC 3953 is about 55 to 60 million light years away, in good agreement with redshift-independent distance estimates of about 30 to 75 million light years. Given that and its apparent size of about 6.6 by 3.1 arcmin (from the images below), the galaxy is about 110 to 115 thousand light years across. NGC 3953 is used by the de Vaucouleurs Atlas of Galaxy Types as an example of type SB(r)bc.

Historical Identification: Per Dreyer, NGC 3954 (= GC 2608 = JH 1013 = WH III 381, 1860 RA 11 46 30, NPD 68 20.7) is "extremely faint, round".
Discovery Notes: Per John Herschel, Dreyer added the following note to the end of the NGC: "(J)h 1013 = (WH) III 381. Marth's identification adopted. The place of III 381 in the catalogue of (Caroline Herschel) from which (J)h's working lists were made out, is vitiated by some great mistake. The (N)PD is supposed to be derived from 1 Comae (Berenices), the neb(ula) being 1³ 12' south of the star. This, however, would give 68³ 9.5' for 1830 instead of 65³ 45.0', that brought up from (Caroline Herschel)."
Physical Information:

Historical Identification: Per Dreyer, NGC 3955 (= GC 2609 = WH II 623, 1860 RA 11 46 54, NPD 112 24.0) is "considerably faint, small, extended 170°±, a little brighter on the south".
Physical Information:

Historical Identification: Per Dreyer, NGC 3956 (= GC 2610 = JH 3368 = WH III 290, 1860 RA 11 46 57, NPD 109 47.5) is "considerably faint, pretty large, pretty much extended 57°".
Physical Information:

Historical Identification: Per Dreyer, NGC 3957 (= GC 2611 = WH II 294, 1860 RA 11 46 58, NPD 108 47.0) is "faint, small, extended, mottled but not resolved".
Physical Information:

Historical Identification: Per Dreyer, NGC 3958 (= GC 2612 = JH 1014 = WH II 833, 1860 RA 11 47 10, NPD 30 51.7) is "pretty faint, pretty small, pretty much extended, very gradually brighter middle".
Physical Information:

Historical Identification: Per Dreyer, NGC 3959 (Tempel list V (#11/12), 1860 RA 11 47 27, NPD 96 58.6) is "very faint, small, between 2 very faint stars".
Physical Information:

Historical Identification: Per Dreyer, NGC 3960 (= GC 2614 = JH 3369, Dunlop 349, 1860 RA 11 47 29, NPD 144 56.4) is "a cluster, pretty large, pretty rich, gradually pretty much brighter middle, stars 13th magnitude".
Physical Information:

Historical Identification: Per Dreyer, NGC 3961 (= GC 2615 = WH III 905, 1860 RA 11 47 31, NPD 19 54.0) is "extremely faint, very small".
Physical Information:

Historical Identification: Per Dreyer, NGC 3962 (= GC 2616 = JH 3370 = WH I 67, 1860 RA 11 47 33, NPD 103 11.8) is "considerably bright, pretty large, irregularly round, gradually much brighter middle, in a triangle with 2 stars".
Physical Information:

Historical Identification: Per Dreyer, NGC 3963 (= GC 2613 = JH 1015 = WH IV 67, 1860 RA 11 47 37, NPD 30 43.9) is "pretty faint, considerably large, round, very gradually then suddenly brighter middle".
Physical Information:

Historical Identification: Per Dreyer, NGC 3964 (= GC 2617 = JH 1016, 1860 RA 11 47 39, NPD 60 57.1) is "very faint, small, extended, 10th magnitude star attached on northeast".
Physical Information:

Historical Identification: Per Dreyer, NGC 3965 (Leavenworth list II (#452), 1860 RA 11 47 58, NPD 100 06.0) is "extremely faint, extremely small, round, brighter middle and nucleus, 9.5 magnitude star 4 arcmin to northwest". The position precesses to RA 11 55 07.0, Dec -10 52 44, but there is nothing there nor "near" there. However, as noted by Corwin, Leander-McCormick observations almost always had right ascensions that were too large, and there is a very faint galaxy 44 seconds of time to the west that perfectly matches the description (including the star to the northwest), so he felt reasonably certain that the galaxy listed above was the correct identification of NGC 3965, providing that Leavenworth could have actually seen such a faint object. Afterward, Steve Gottlieb used a telescope of similar size to Leavenworth's and stated that he could immediately see the galaxy and that it perfectly matched the description; and images of the galaxy show that it has a high surface brightness, so despite its small size and brightness there is essentially no doubt that PGC 157086 is indeed what Leavenworth photographed.
NED Designation Note: NED generally ignores "PGC" objects not in the original Principal Galaxy Catalog, so a search for this object should use LEDA 157086.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 15015 km/sec (and H₀ = 70 km/sec/Mpc), a straightforward calculation indicates that NGC 3965 is about 700 million light years away, in good agreement with a single redshift-independent distance estimate of about 675 to 680 million light years. However, for objects at such distances we should take into account the expansion of the Universe during the time it took their light to reach us. Doing that shows that the galaxy was about 660 million light years away at the time the light by which we see it was emitted, about 675 million years ago (the difference between the two numbers being due to the expansion of the intervening space during the light-travel time). Given that and its apparent size of about 0.35 by 0.2 arcmin (from the images below), the galaxy is about 65 to 70 thousand light years across.

Historical Identification: Per Dreyer, NGC 3966 (= GC 5592, d'Arrest, 1860 RA 11 47 59, NPD 57 01.5) is "faint, pretty large, a little extended, brighter middle, 12th magnitude star to west". The second IC lists a corrected RA (per Wolf list VIII #187) of 11 48 29, and notes that d'Arrest made only one observation. Mostly ignoring the IC2 note for the moment (which, as discussed in the Discovery Note below was an unfathomable blunder on Wolf's part), the original NGC position precesses to RA 11 55 13.7, Dec +32 11 46, but there is nothing there nor near there save for (well off to the east) the extremely faint object which Wolf mistook for NGC 3966, and cannot possibly be what d'Arrest saw. The key to the problem (as noted by Corwin) is d'Arrest's comment, "Found while looking for (what became NGC 3986); this is either a nova, or my RA is inexplicably erroneous." d'Arrest's RA is indeed erroneous, being off by a minute and a half of time, and his declination is also off, by 10 arcmin. Such a double error in the position is practically unheard of for d'Arrest, but his description is absolutely perfect for NGC 3986, right down to the (double) star west of the galaxy's nucleus and south of the western arm of its disk, and since the "nebula" that became NGC 3986 is what he was looking for, it is virtually certain that that is indeed what he did observe, but for some reason (as he suspected) he made an inexplicable error in his position, leading to the duplicate entry.
Discovery Note: Dreyer's IC2 note does not give Wolf's entire position for his list VIII #187, which was 1875 RA 11 49 15.2, NPD 57 06 56, nor his description of the object as a small, pretty bright planetary nebula (which, considering the object he presumably observed, is completely inaccurate, save for "small"). The position precesses to 1860 RA 11 48 28.5, NPD 57 01 56, or J2000 RA 11 55 42.9, Dec +32 11 20. This falls just outside the eastern rim of PGC 37462, which is generally listed as IC 2981, not NGC 3966 (though I have seen several references which incorrectly list or label that object as NGC 3966 as a result of Wolf's mistake), so Wolf's "identification" of this object as NGC 3966 was actually a misidentification, which is hardly surprising, since the object at Wolf's position has nothing in common with d'Arrest's description of what he observed (as already noted above), and how Wolf could have thought it was NGC 3966 is almost beyond understanding. However, d'Arrest's position is so bad that Wolf's list VIII #187 lies only a few arcmin to the east of it, and Wolf must have considered the "reasonable" agreement in position more important than the completely incompatible description of the object. [Additional Note: As it happens, Wolf did observe NGC 3966/3986, as shown in the discovery credits above, but as his list VIII #232 (correctly identified by him as NGC 3986).]
Note About The NGC Designation: Standard practice for duplicate entries in the NGC is to use the lower numeric designation, even if it is badly flawed, and a higher numeric designation is perfectly accurate; so usually, this galaxy would be listed as NGC 3966, and in fact most of the catalogs which correctly identify it as a duplicate of NGC 3986 do call it NGC 3966. But since so many other references incorrectly list NGC 3966 as Wolf's mistaken identification, I have chosen to deal with the actual galaxy in the entry for NGC 3986, and to restrict this entry to the problems with the identification of NGC 3966.
Physical Information: Given the duplicate entry, see NGC 3986 for anything else.

Historical Identification: Per Dreyer, NGC 3967 (Temple list V (#13/14), 1860 RA 11 48 01, NPD 97 03.9) is "very faint, small, faint star close to west".
Physical Information:

Historical Identification: Per Dreyer, NGC 3968 (= GC 2621 = JH 1018 = WH II 162, 1860 RA 11 48 15, NPD 77 15.1) is "pretty bright, large, irregularly round, brighter middle, 10th magnitude star 3 arcmin distant at position angle 65°".
Physical Information:

Historical Identification: Per Dreyer, NGC 3969 (Ormond Stone list II (#453), 1860 RA 11 48 17, NPD 107 58.0) is "extremely faint, very small, gradually brighter middle and nucleus, 10th magnitude star 4 arcmin to northwest". The second IC adds (per Howe) "RA 11 48 01, NPD 108 08.9, the star is 8.5 magnitude, and is nearly north".
Physical Information:

Historical Identification: Per Dreyer, NGC 3970 (= GC 2623 = JH 1020, 1860 RA 11 48 19, NPD 101 15.9) is "faint, small, round, pretty suddenly brighter middle, western of 2", the other being NGC 3974.
Physical Information:

Historical Identification: Per Dreyer, NGC 3971 (= GC 2624 = JH 1019 = WH II 724, 1860 RA 11 48 20, NPD 59 13.7) is "pretty faint, very small, round, brighter middle".
Physical Information:

Historical Identification: Per Dreyer, NGC 3972 (= GC 2618 = WH II 789, d'Arrest, 1860 RA 11 48 24, NPD 33 54.7) is "pretty bright, extended".
Discovery Notes: In the notes at the end of the NGC Dreyer wrote the following for NGC 3972 and 3977: "(WH) II 789 - 790. d'A(rrest) saw (only the) p(receding = western) one, but I saw both in 1878. The place of II 790 in the (NG)Catalogue was found by combining d'A(rrest)'s place for II 789 with my measure of Pos(ition) and Dist(ance) between them."
Physical Information: NGC 3972 is used by the de Vaucouleurs Atlas of Galaxy Types as an example of type SA(s)bc.

Historical Identification: Per Dreyer, NGC 3973 (= GC 2622, 3rd Lord Rosse, 1860 RA 11 48 24, NPD 77 13) is "extremely faint, extremely small, 10th magnitude star 1 arcmin to southeast (requires verification)".
Discovery Notes: (standard paragraph about 3rd Lord Rosse's assistants)
Physical Information:

Historical Identification: Per Dreyer, NGC 3974 (= GC 2625 = JH 1021, 1860 RA 11 48 32, NPD 101 12.7) is "very faint, small, round, brighter middle, eastern of 2", the other being NGC 3970.
Physical Information:

Historical Identification: Per Dreyer, NGC 3975 (= GC 5593, 4th Lord Rosse, 1860 RA 11 48 33, NPD 28 41.5) is "very faint, very small, II 840 seventeen seconds of time to the east", "(WH) II 840" being NGC 3978.
Physical Information:

Historical Identification: Per Dreyer, NGC 3976 (= GC 2626 = JH 1022 = WH II 132, 1860 RA 11 48 47, NPD 82 28.3) is "bright, pretty large, considerably extended 30°, very suddenly much brighter middle and nucleus".
Discovery Notes: Palisa's observation is on page 349 of Steinicke's S360, and not only gives the (1886) position but also states that the object is GC 2626.
Physical Information:

Historical Identification: Per Dreyer, NGC 3977 (= GC 2619 = WH II 790, Dreyer, 1860 RA 11 48 48, NPD 33 50.4) is "faint, small".
Discovery Notes: In the notes at the end of the NGC Dreyer wrote the following for NGC 3972 and 3977: "(WH) II 789 - 790. d'A(rrest) saw the only p(receding = western) one, but I saw both in 1878. The place of II 790 is the (NG)Catalogue was found by combining d'A(rrest)'s place for II 789 with my measure of Pos(ition) and Dist(ance) between them."

Physical Information:

Historical Identification: Per Dreyer, NGC 3978 (= GC 2627 = JH 1023 = WH II 840, 1860 RA 11 48 50, NPD 28 42.0) is "considerably faint, small, a little extended, brighter middle, 8th magnitude star 6 arcmin distant at position angle 90°".
Physical Information:

Historical Identification: Per Dreyer, NGC 3979 (Holden (#6), Swift list III (#??), 1860 RA 11 48 55, NPD 91 55.0) is "pretty faint, 11th or 12th magnitude star to northeast".
Physical Information:

Historical Identification: Per Dreyer, NGC 3980 (Swift list I (#18), 1860 RA 11 48 56, NPD 33 50.0) is "extremely faint, pretty large, extended, double star near".
Physical Information:

Historical Identification: Per Dreyer, NGC 3981 (= GC 2628 = WH III 274, 1860 RA 11 49 04, NPD 109 07.0) is "very faint, pretty large, irregular figure". The position precesses to RA 11 56 12.2, Dec -19 53 45, less than 1.2 arcmin east of the galaxy listed above, the description fits and there is nothing else nearby, so the identification is certain.
Physical Information: Based on a recessional velocity of 1725 km/sec (and H₀ = 70 km/sec/Mpc), NGC 3981 is about 80 million light years away, in good agreement with redshift-independent distance estimates of 50 to 95 million light years (the ESO image discussion lists a distance of 65 million light years). Given that and its apparent size of 2.9 by 0.6 arcmin (from the images below), the bright central part of the galaxy is about 65 to 70 thousand light years across, while its extended arms, which have an apparent size of roughly 7 by 5.5 arcmin, span about 160 to 165 thousand light years. The extensions suggest some kind of interaction with another galaxy, long enough ago that there is now nothing close enough to be an obvious candidate for such an interaction. However, NGC 3981 is part of the NGC 4038 group, named after one of the pair of galaxies called The Antennae, so some member of the group is almost certainly the cause of the unusual structure seen here.

Historical Identification: Per Dreyer, NGC 3982 (= GC 2620 = JH 1017 = WH IV 62, d'Arrest, Schultz, 1860 RA 11 49 09, NPD 34 05.8) is "bright, pretty large, round, gradually then suddenly brighter middle and disc".
Physical Information:

Historical Identification: Per Dreyer, NGC 3983 (= GC 2629 = JH 1024 = WH III 343, 1860 RA 11 49 10, NPD 65 20.9) is "considerably faint, considerably small, round, pretty suddenly brighter middle".
Physical Information:

Historical Identification: Per Dreyer, NGC 3984 (= GC 2630 = JH 1026, 1860 RA 11 49 23, NPD 60 13.4) is "extremely faint, small, round, brighter middle". The second IC says "3984: Nothing in this place, Wolf list VIII (h. one observation, about which he seemed rather doubtful)".
Physical Information:

Historical Identification: Per Dreyer, NGC 3985 (= GC 2631 = JH 1025 = WH III 707, 1860 RA 11 49 24, NPD 40 53.0) is "very faint, considerably small, another suspected".
Physical Information:

Historical Identification: Per Dreyer, NGC 3986 (= GC 2632 = JH 1027, 1860 RA 11 49 30, NPD 57 12.0) is "pretty faint, small, pretty much extended 90°±, 11th magnitude star near". The position precesses to RA 11 56 43.8, Dec +32 01 15, essentially dead center on the galaxy listed above, the description fits (though the "11th magnitude star" is actually a fairly close double) and there is nothing comparable nearby, so the identification is certain. (See NGC 3966 for a discussion of the supposed duplicate entry.)
Discovery Note: Although Max Wolf had nothing to do with the identification of NGC 3986, he did observe it (as his list VIII #232) and hundreds of other nebulae on a plate taken on Mar 28, 1906, and his list VIII #178 is the one that he mistakenly identified as NGC 3966, with the result that although NGC 3966 is actually an "inexplicably erroneous" observation of NGC 3986 by d'Arrest, it is often listed as Wolf's completely mistaken identification of what is actually IC 2981. Given that substantial contribution to modern confusion about the identity of NGC 3966, and the fact that NGC 3966 is actually a duplicate observation of NGC 3986, it seems appropriate to mention Wolf's observations here.
Discussion of NGC Designation: Although the identification of NGC 3986 is certain, and that of NGC 3966 is only nearly certain, the general consensus is that the two NGC entries represent the same object. As a result, the usual practice of using the smaller NGC designation means that most catalogs refer to this galaxy as NGC 3966. As a result, it is possible that this entry will only retain a historical discussion, and everything else will be moved to the entry for NGC 3966.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 3545 km/sec (and H₀ = 70 km/sec/Mpc), NGC 3986 is about 165 million light years away, in fair agreement with a single redshift-independent distance estimate of about 190 million light years. Given that and its apparent size of about 2.7 by 0.5 arcmin (from the images below), NGC 3986 is about 130 thousand light years across. (Note: The galaxy type is usually listed as a lenticular galaxy of type S0/a. Since it is edge-on, its actual type is hard to determine, but given its very flattened disk, obvious dust lanes and large nuclear bulge, I think that it is almost certainly a type Sa spiral, though I have added "peculiar" because of the curvature of its disc and a hint of complex structure in the nucleus, as shown in the description at top.)

Historical Identification: Per Dreyer, NGC 3987 (= GC 2638, 3rd Lord Rosse, 1860 RA 11 50 07, NPD 64 01.5) is "faint, much extended". The position precesses to RA 11 57 19.7, Dec +25 11 45, essentially dead center on the galaxy listed above, the description fits and a comparison of the sketch shown below to the sky makes the identification certain.
Discovery Notes (1): Although Dreyer credited the discovery to William Parsons, 3rd Earl of Rosse, in the NGC he noted that many of Rosse's nebular discoveries were actually made by one of his assistants, in this case R. J. Mitchell. In Dreyer's chart of the region (shown below) NGC 3987 is labeled as θ.
Discovery Notes (2): The later-added discovery credit for Herschel is based on Dreyer's notes for the 1912 Scientific Papers of Sir William Herschel, in which he wrote that WH III 323's position (as calculated by Auwers) was in excellent accordance with θ in his sketch of observations done at Birr Castle, which is NGC 3987, while the position for (WH) III 324 corresponded to η (or NGC 3993) in the same diagram.

 Above, a sketch drawn by John Dreyer of the region near JH 1033 (= NGC 3997), as shown on page 105 of Lawrence Parsons' (the 4th Lord Rosse) 1881 publication of observations done with his father's 72-inch "Leviathan" by him, his father (William Parsons, the 3rd Lord Rosse), and their assistants (in this case, R. J. Mitchell and John Dreyer. NGC 3987, 3989, 3993, 3997 and 4005 were observed or discovered by R. J. Mitchell in 1854, NGC 3999 and 4000 were discovered by the 4th Lord Rosse in 1878, and NGC 4009, 4011, 4015, 4018, 4021, 4022 and 4023 were discovered by John Dreyer in 1878. Dreyer drew the arrow pointing westward, all the positions of stars and nebulae, and assigned the Greek letters for the nebulae; I added the NGC designations, the Northward arrow and the labels for Northward and Westward.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 4810 km/sec (and H₀ = 70 km/sec/Mpc), NGC 3987 is about 225 million light years away, in good agreement with redshift-independent distance estimates of about 135 to 245 million light years. Given that and its apparent size of about 2.4 by 0.4 arcmin (from the images below), the galaxy is about 155 to 160 thousand light years across.

Historical Identification: Per Dreyer, NGC 3988 (= GC 2633 = JH 1028, 1860 RA 11 50 10, NPD 61 20.7) is "very faint, small, round, brighter middle like a star, western of 2", the other almost certainly being NGC 4004, but not specified as such in the NGC.
Physical Information:

Historical Identification: Per Dreyer, NGC 3989 (= GC 2639, 3rd Lord Rosse, 1860 RA 11 50 12, NPD 63 58±) is "extremely faint, very small, round". The position precesses to RA 11 57 24.7, Dec +25 15 15, about 1.4 arcmin north northwest of the galaxy listed above, but the description fits and a comparison of the sketch shown below to the sky makes the identification certain.
Discovery Notes: Although Dreyer credited the discovery to William Parsons, 3rd Earl of Rosse, in the NGC he noted that many of Rosse's nebular discoveries were actually made by one of his assistants, in this case R. J. Mitchell. In Dreyer's chart of the region (shown below) NGC 3989 is labeled as ξ.

 Above, a sketch drawn by John Dreyer of the region near JH 1033 (= NGC 3997), as shown on page 105 of Lawrence Parsons' (the 4th Lord Rosse) 1881 publication of observations done with his father's 72-inch "Leviathan" by him, his father (William Parsons, the 3rd Lord Rosse), and their assistants (in this case, R. J. Mitchell and John Dreyer. NGC 3987, 3989, 3993, 3997 and 4005 were observed or discovered by R. J. Mitchell in 1854, NGC 3999 and 4000 were discovered by the 4th Lord Rosse in 1878, and NGC 4009, 4011, 4015, 4018, 4021, 4022 and 4023 were discovered by John Dreyer in 1878. Dreyer drew the arrow pointing westward, all the positions of stars and nebulae, and assigned the Greek letters for the nebulae; I added the NGC designations, the Northward arrow and the labels for Northward and Westward.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 5020 km/sec (and H₀ = 70 km/sec/Mpc), NGC 3989 is about 230 to 235 million light years away. Given that and its apparent size of about 0.65 by 0.3 arcmin (from the images below), the galaxy is about 45 thousand light years across.

Historical Identification: Per Dreyer, NGC 3990 (= GC 2634 = JH 1029 = WH II 791, 1860 RA 11 50 17, NPD 33 45.7) is "pretty faint, small, a little extended, pretty suddenly a little brighter middle".
Physical Information:

Historical Identification: Per Dreyer, NGC 3991 (= GC 5594, d'Arrest, 1860 RA 11 50 18, NPD 56 52.4) is "faint, small, a little extended, 1st of 3", the others being NGC 3394 and 3395.
Physical Information: (Corwin lists two positions: RA 11 57 31.7, Dec +32 20 29, and RA 11 57 30.4, Dec +32 20 03; so perhaps this is a double?)

Historical Identification: Per Dreyer, NGC 3992 (= GC 2635 = JH 1030 = WH IV 61, 1860 RA 11 50 19, NPD 35 51.3) is "considerably bright, very large, pretty much extended, suddenly brighter middle and bright mottled but not resolved nucleus". The position precesses to RA 11 57 36.0, Dec +53 21 57, barely south of the nucleus of the galaxy and well within its outline, there is nothing else nearby and the description fits, so the identification is certain.
Discovery Note (1): Méchain's observation of Mar 12, 1781 was long thought to be the first observation of this object, but as of his letter of May 6, 1783 to Bernoulli he had still not determined its position, and in 2006 Henrik Bril found that based on the position of the nebula near Phecda (γ Ursae Majoris) on plates 6 and 7 of the 1795 edition of the Atlas Fortin-Flamsteed, updated and edited by Lalande and Méchain, what Méchain almost certainly observed was NGC 3953 (I have examined Bril's work and plate 6 and agree with this assessment; but see the Designation Note below). Sometime soon after (in late March, April or early May of 1781) Messier observed both NGC 3953 and 3992, and recorded a handwritten position in his personal copy of his Catalog that has the right ascension of NGC 3953, but the declination of NGC 3992. That position is obviously not terribly good, but since its declination agrees with that of NGC 3992, and declinations were far easier for early observers to accurately record than right ascensions, it is reasonably certain that Messier did observe what became NGC 3992, and on that basis Owen Gingerich added it to the Messier Catalog as M109. Since Messier did not add it to his published Catalog, neither the Herschels nor Dreyer were aware of its prior observation when compiling their various catalogs, causing the lack of any mention of Méchain or Messier in the NGC.
Discovery Note (2): Stoney's observation and sketch of NGC 3992 (using the 3rd Lord Rosse's 72-inch "Leviathan") was not meant to be a study of that galaxy, but of the region surrounding it; and as a result of that he discovered PGC 2412642; but since that object never became an NGC or IC object, it is not covered here but in the entry linked to its PGC designation.
Designation Note: Bril asserts that since Méchain had been thought to be the original discoverer of M109 (= NGC 3992), and what he actually observed was not that object but NGC 3953, that galaxy should be called M109 (hence the need to deny that possibility in the entry for NGC 3953). However, the reason that Gingerich added M108 and M109 to the Messier Catalog was that Messier observed them, and whether or not Méchain also observed them was irrelevant. All that Bril's discovery does is to remove Méchain from the list of observers of M109, and add him to the list of observers of NGC 3953.
Physical Information: Based on a recessional velocity of 1050 km/sec, NGC 3992 is about 50 million light years away, in reasonable agreement with widely varying redshift-independent distance estimates of 35 to 115 million light years. Given that and its apparent size of 7.3 by 4.2 arcmin, it is about 100 thousand light years across. NGC 3992 is used by the de Vaucouleurs Atlas of Galaxy Types as an example of type SB(rs)bc.

Historical Identification: Per Dreyer, NGC 3993 (= GC 2640, 3rd Lord Rosse, 1860 RA 11 50 25, NPD 63 58.8) is "very faint, pretty small, extended, 3 stars near". The position precesses to RA 11 57 37.6, Dec +25 14 27, essentially dead center on the galaxy listed above, the description fits and a comparison of the sketch shown below to the sky makes the identification certain.
Discovery Notes (1): Although Dreyer credited the discovery to William Parsons, 3rd Earl of Rosse, in the NGC he noted that many of Rosse's nebular discoveries were actually made by one of his assistants, in this case R. J. Mitchell. In Dreyer's chart of the region (shown below) NGC 3993 is labeled as η.
Discovery Notes (2): The later-added discovery credit for Herschel is based on Dreyer's notes for the 1912 Scientific Papers of Sir William Herschel, in which he wrote that WH III 323's position (as calculated by Auwers) was in excellent accordance with θ in his sketch of observations done at Birr Castle, which is NGC 3987, while the position for (WH) III 324 corresponded to η (or NGC 3993) in the same diagram.

 Above, a sketch drawn by John Dreyer of the region near JH 1033 (= NGC 3997), as shown on page 105 of Lawrence Parsons' (the 4th Lord Rosse) 1881 publication of observations done with his father's 72-inch "Leviathan" by him, his father (William Parsons, the 3rd Lord Rosse), and their assistants (in this case, R. J. Mitchell and John Dreyer. NGC 3987, 3989, 3993, 3997 and 4005 were observed or discovered by R. J. Mitchell in 1854, NGC 3999 and 4000 were discovered by the 4th Lord Rosse in 1878, and NGC 4009, 4011, 4015, 4018, 4021, 4022 and 4023 were discovered by John Dreyer in 1878. Dreyer drew the arrow pointing westward, all the positions of stars and nebulae, and assigned the Greek letters for the nebulae; I added the NGC designations, the Northward arrow and the labels for Northward and Westward.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 5135 km/sec (and H₀ = 70 km/sec/Mpc), NGC 3993 is about 240 million light years away, in good agreement with redshift-independent distance estimates of about 200 to 280 million light years. Given that and its apparent size of about 1.65 by 0.4 arcmin (from the images below), NGC 3993 is about 115 thousand light years across.

Historical Identification: Per Dreyer, NGC 3994 (= GC 5595, d'Arrest, 1860 RA 11 50 25, NPD 56 56.1) is "pretty bright, very small, 2nd of 3", the others being NGC 3391 and 3395.
Physical Information:

Historical Identification: Per Dreyer, NGC 3995 (= GC 5596, d'Arrest, 1860 RA 11 50 32, NPD 56 55.0) is "faint, pretty large, irregularly round, brighter middle, 3rd of 3", the others being NGC 3391 and 3394.
Physical Information:

Historical Identification: Per Dreyer, NGC 3996 (= GC 2636 = JH 1032, 1860 RA 11 50 34, NPD 74 55.5) is "very faint, pretty large, round, 2 stars to east".
Physical Information:

Historical Identification: Per Dreyer, NGC 3997 (= GC 2641 = JH 1033, 1860 RA 11 50 36, NPD 63 57.0) is "pretty faint, very small, extended 25°, between 2 stars". The position precesses to RA 11 57 48.6, Dec +25 16 15, essentially dead center on the galaxy listed above, the description fits and a comparison of the sketch shown below to the sky makes the identification certain.
Discovery Notes: In Dreyer's chart of the region (shown below) NGC 3997 is labeled as ζ.

 Above, a sketch drawn by John Dreyer of the region near JH 1033 (= NGC 3997), as shown on page 105 of Lawrence Parsons' (the 4th Lord Rosse) 1881 publication of observations done with his father's 72-inch "Leviathan" by him, his father (William Parsons, the 3rd Lord Rosse), and their assistants (in this case, R. J. Mitchell and John Dreyer. NGC 3987, 3989, 3993, 3997 and 4005 were observed or discovered by R. J. Mitchell in 1854, NGC 3999 and 4000 were discovered by the 4th Lord Rosse in 1878, and NGC 4009, 4011, 4015, 4018, 4021, 4022 and 4023 were discovered by John Dreyer in 1878. Dreyer drew the arrow pointing westward, all the positions of stars and nebulae, and assigned the Greek letters for the nebulae; I added the NGC designations, the Northward arrow and the labels for Northward and Westward.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 5080 km/sec (and H₀ = 70 km/sec/Mpc), NGC 3997 is about 235 to 240 million light years away, in good agreement with redshift-independent distance estimates of about 240 to 275 million light years. Given that and its apparent size of about 1.7 by 1.15 arcmin (from the images below), the galaxy is about 115 to 120 thousand light years across. The galaxy's peculiar structure, undoubtedly due to some kind of relatively recent disturbance, includes substantial star-forming regions in and near its nucleus and several diffuse structures in addition to its exceptionally broad outer arms.

Historical Identification: Per Dreyer, NGC 3998 (= GC 2637 = JH 1031 = WH I 229, 1860 RA 11 50 38, NPD 33 46.0) is "considerably bright, pretty small, round, very gradually then suddenly much brighter middle".
Physical Information: Apparent size 2.7 by 2.3 arcmin. NGC 3998 is used by the de Vaucouleurs Atlas of Galaxy Types as an example of galaxy type SA(r)0°.

Historical Identification: Per Dreyer, NGC 3999 (4th Lord Rosse, 1860 RA 11 50 44, NPD 64 09.4) is "very faint, small". The position precesses to RA 11 57 56.5, Dec +25 03 51, only 0.2 arcmin south of the galaxy listed above, the description fits and a comparison of the sketch shown below to the sky makes the identification certain.
Discovery Notes: In Dreyer's chart of the region (shown below) NGC 3999 is labeled as μ.

 Above, a sketch drawn by John Dreyer of the region near JH 1033 (= NGC 3997), as shown on page 105 of Lawrence Parsons' (the 4th Lord Rosse) 1881 publication of observations done with his father's 72-inch "Leviathan" by him, his father (William Parsons, the 3rd Lord Rosse), and their assistants (in this case, R. J. Mitchell and John Dreyer. NGC 3987, 3989, 3993 and 4005 were observed or discovered by R. J. Mitchell in 1854, NGC 3999 and 4000 were discovered by the 4th Lord Rosse in 1878, and NGC 4009, 4011, 4015, 4018, 4021, 4022 and 4023 were discovered by John Dreyer in 1878. Dreyer drew the arrow pointing westward, all the positions of stars and nebulae, and assigned the Greek letters for the nebulae; I added the NGC designations, the Northward arrow and the labels for Northward and Westward.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 4465 km/sec (and H₀ = 70 km/sec/Mpc), NGC 3999 is about 205 to 210 million light years away. Given that and its apparent size of about 0.5 by 0.2 arcmin (from the images below), the galaxy is about 30 thousand light years across.