1. A description and several fine photographs of NGC 6231 may be conveniently found in Burnham RobertJr, Burnham's celestial handbook (3 vols, New York, 1978), iii, 1722–6.
2. See for example Jones Glyn Kenneth, The search for the nebulae (Chalfont St Giles, 1975), 78, no. 53. Under the heading “Nos in other Catalogues”, Glyn Jones references for NGC 6231 only Lac.II.13, referring to Lacaille's 1755 catalogue. Checking older sources, we find that John Herschel, in his “Catalogue of nebulae”, Philosophical transactions of the Royal Society of London, cliv (1864), 122, no. 4245, cites as authority only Dunlop, as Δ499. Turning to James Dunlop's nebulae catalogue, ibid., cxviii (1828), 139, no. 499, we are referred only to Bode, and consulting Bode's Allgemeine Beschreibung und Nachweisung der Gestirne (Berlin, 1801), 58, no. 150, we are informed that the cluster was observed by Lacaille. The trail stops at Lacaille. We shall discuss Lacaille's observation in more detail below.