What was the Brocken spectre?

In Loudon’s 1848 account, a traveler on the Brocken may see a magnified, distorted image of themselves on a cloud opposite them. She calls it the “spectre of the Brocken” and says the image is reflected there “under particular circumstances,” without explaining those circumstances further. The book says the best time to see it is September, around four in the afternoon^[1].

Loudon treats that image separately from the local beliefs she records: country people regarded the Brocken’s summit as an annual meeting place for Germany’s witches and spirits, and the wild huntsman was also supposed to dwell there. Those are presented as beliefs and legends, not as her account of what produces the spectre^[2].