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I am quoting here verbatim from John Herschel's Cape Observations, which has a large section devoted to the Eta Carina Nebula and of course the outburst of Eta Car that he chronicled.
"Close to the great star A, is situated that singular lemniscate-oval vacuity which forms so strange a feature of this nebula. Its area is not entirely devoid of light. A thin nebulous veil seems as if extended over its northern loop on the preceding side. Four stars, Nos. 686, 603, 589, and 670 (= [w]) of the Catalogue are placed precisely on its edges, and will serve as excellent detectors of change in its form, should any occur. The stars Nos. 607 (=[t]), 664 (=[v]), and 616, though near the edge, are yet fairly immersed in the nebula. On the other hand [u] No. 634, situated in the contraction of the oval towards its middle, though also near the edge, is yet fairly within the vacancy, and so situated that the slightest shifting of the nebulous contour at its preceding side cannot fail to rendered sensible."
Herschel's gorgeous sketch of the Eta Car nebula clearly shows a very distinctive guitar or keyhole-shaped vacuity in the nebula (much more contrasty than visible today), but I couldn't find any phrase "Keyhole" in his description. Instead he calls it a lemniscate-oval vacuity. Of course, the "Lemniscate-oval Vacuity Nebula" doesn't exactly roll off the tongue, so I'm wondering if this was just abbreviated into the Keyhole Nebula later?
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