The Brocken Spectre appears when a low sun is behind a climber who is looking downwards into mist from a ridge or peak. The "spectre" is the shadowy figure - the glow and rings are of course a glory centered
directly opposite the sun at the antisolar point. But how is the ghostly figure produced? It is no more than the shadow of the climber projected forward through the mist. All shadows converge towards the antisolar point where the glory also shines. The sometimes odd triangular shape is a perspective effect. The Brocken Spectre is a similar effect to anti-crepuscular rays and cloud shadows.
Dear A. Bouazza,
This "Brocken Spectre" figures prominently in Hogg's Confessions, Coleridge ("cedarn") traveled to the Harz mountains in Germany in hopes of experiencing the optical illusion, and Shade's parahelia suggest that VN too found it an interesting phenomenon.
Carolyn
p.s. I haven't read the novel, but I wonder if "Glory" doesn't refer to the optical effect?