Hitchcock’s Vertigo is about loss; the loss of identity; love; and sanity. Based on a French novel by Pierre Boileau and Thomas Narcejac the movie, however, alters the structure (along with other differences) thus changing it from a mystery (the novel had a twist ending which Hitchcock gives away two thirds of the way through) to that of a suspenseful, tragic love story.
Hitchcock achieves his goals on one level by the use of costumes. It is through costumes Judy loses her identity; first to Elster for money and then to Scottie for love. These same costumes make Scottie lose his sanity as he sees his beloved Madeline in anyone that appears to wear similar clothes as she did and then later in his insane obsession of changing Judy into Madeline in the hope of resurrecting a lost love. And it is Carlotta’s necklace which Judy kept that finally pulls the wool from Scottie’s eyes and releases the darkness within him (quite brilliantly played by James Stewart).