Nimbus and niche
Near the end of Telemachus, an old man pops to the surface of the swimming hole and clambers up the rocks, seawater running off of his body and bathing trunks. Glancing at Stephen and Haines, Mulligan crosses himself, to indicate that the swimmer is a priest. His information is confirmed later when the narrative mentions "The priest's grey nimbus in a niche where he dressed discreetly." The nimbus seems to refer to a "garland of grey hair" seen earlier on the old man, and the niche to a gap between rocks on the shore where he can modestly remove his swimsuit, but both words have religious overtones.
"Nimbus" (Latin for cloud) is a plausibly descriptive detail for a small bit of hair seen from a distance. In the works of antiquity, gods often concealed themselves in dark clouds or haloes of light when they appeared to human beings. Virgil’s Aeneid frequently shows the gods hiding themselves from the men whom they favor in clouds. "Niche," in Catholic usage, refers to a shallow architectural recess (e.g., a semicircular space in the wall of a medieval cathedral) designed to hold a statue (e.g., of a saint).
Gifford suggests that the man’s “garland of grey hair” just before Mulligan’s gesture indicates that he has a tonsure, but the phrase might just as easily describe the ear-level ring of hair that many balding men acquire naturally. In Proteus Stephen recalls the "garland of grey hair" as he thinks of the supposedly bald Joachim of Fiore.