(Credit: © Marc Brenner/Royal Opera House) In “Alcina” seeing isn’t believing. Handel’s tale of an enchanted island, which sees the titular sorceress’ captives transfigured into birds, beasts, and shrubbery, speaks of the pleasures and dangers of illusion. First premiered on the current site of the Royal Opera House back in 1735, the opera speaks to a host of 18th-century preoccupations {…}