Definition of "arcana"

Thou deign'st no answer,—or I fain would askIf since thy bright creation, thou hast seenOught like a Newton, whose admitted eyeThe arcana of the universe exploredLight's subtle ray its mechanism disclosed,The impetuous comet his mysterious loreUnfolded,

1827, Lydia Sigourney, Poems, To the Moon, page 15

This is not, however, a mere salmagundi of alphabetical arcana.

2013 September 14, Jane Shilling, “The Golden Thread: the Story of Writing, by Ewan Clayton, review [print edition: Illuminating language]”, in The Daily Telegraph (Review)‎, page R29