It recently occurred to me that, inspite of frequent list-headings on a "third man", Shackleton's writings describe three men crossing the ice, while the brocken shadow came as a "fourth" presence.  So, actually, what we find is a "Fourth Man".  The third man comes from Eliot  (The Waste Land: Who is the third who walks always beside you?)  In Shackleton we find: "I know that during that long, and racking march ...it seemed to me often that we were four, not three". One of them was Shackleton himself, the other two were described as Worsley and Crean.

The same conclusion pertains to H. Bosch's "Triptych" ( The Garden of Heavenly and Earthly Delights) that reveals three painted panels when its two lids are open, and a fourth image when it is closed, like VN's material book in print and, perhaps, its afterimage after we close our eyes.

So, we have in Canto One:  I was the shadow of the waxwing slain/  I was the smudge of ashen fluff — and I lived on, flew on, in the reflected sky. ( three "I")

whereas Shade also mentions: And from the inside, too, I’d duplicate Myself/  ( a "fourth", a reflection on dark glass)

He then invites us to a cinematic: "Retake the falling snow: each drifting flake..." until he intimates Gradus ( All colors made me happy: even gray.) before mentioning (line 30 and on) that:" My eyes were such that literally they/  Took photographs. Whenever I’d permit, Or, with a silent shiver, order it, Whatever in my field of vision dwelt ... Was printed on my eyelids’ nether side/ Where it would tarry for an hour or two, And while this lasted all I had to do/ Was close my eyes to reproduce the leaves..."

About a triptych: The direct reference comes from Kinbote's commentaries for line 70 ( where King Alfin cum airplane crashes against a building and me find a "miragarl" in Zembla and "three fountains", or "phantana" in Zemblan):

He awoke to find her standing with a comb in her hand before his — or rather, his grandfather’s — cheval glass, a triptych of bottomless light, a really fantastic mirror, signed with a diamond by its maker, Sudarg of Bokay. She turned about before it: a secret device of reflection gathered an infinite number of nudes in its depths, garlands of girls in graceful and sorrowful groups, diminishing in the limpid distance, or breaking into individual nymphs, some of whom, she murmured, must resemble her ancestors when they were young — little peasant garlien combing their hair in shallow water as far as the eye could reach, and then the wistful mermaid from an old tale, and then nothing.