Dramatic irony is a literary device whereby the reader knows something that the characters do not. At the same time, we cannot know too much, as that would spoil the ending. So a skillful author like Edith Wharton drops little hints of irony like breadcrumbs throughout a story, allowing us to build up a gradual picture of what is really happening.
The story's setting seems perfectly conventional. A couple of wealthy, middle-aged American ladies are spending what appears to be a perfectly pleasant time together in Rome, overlooking the Forum. But this particular scene is almost too perfect, too serene. Right from the outset, we are aware that there is something one or both of these ladies does not know, even if we are not quite sure what it is. Wharton's famed skill in penetrating the prim facade of high society to expose its underlying tensions immediately puts us on our guard, leading us to expect the unexpected.
And we are not to be disappointed. In the figure of Mrs. Slade we are confronted by a thoroughly unpleasant individual, someone we would do well to avoid. In contrast, the unprepossessing Mrs. Ansley suggests hidden depths beneath her mousy exterior. It is much too early in the story to speculate, but we do sense something is going on between them.
The incipient sense of dramatic irony is fleshed out in a throwaway remark by Mrs. Slade, which, in addition to being laden with irony, displays foreshadowing:
And I was wondering, ever so respectfully, you understand... wondering how two such exemplary characters as you and Horace had managed to produce anything quite so dynamic.
How on earth could two such dullards (in Mrs. Slade's opinion) produce such a bright, vivacious young creature as Barbara? It is an absolute mystery to Mrs. Slade.
And when Mrs. Slade (again, casually) starts talking about how lovers used to conduct their little trysts in the Colosseum, we feel ourselves on the precipice of a shocking revelation. Mrs. Slade is not a tourist guide nor is she just making idle conversation; she clearly has a reason for mentioning this. Mrs. Ansley plays innocent, giving the impression that she is not really interested in the subject, carrying on with the insistent click-clack of her knitting. But she protests too much, and we are not convinced. Lovers? Trysts? The Colosseum? We are now starting to put two and two together, and we have a vague idea as to what this little charade is all about. But the dramatic irony of the developing situation compels us to recognize that Mrs. Slade's brief history lesson has profound implications for her too, not just Mrs. Ansley.
When the truth is finally revealed—the letter, the secret tryst, the identity of Barbara's biological father—dramatic irony provides a satisfying and well-wrought conclusion to the story. Wharton artfully puts together the small but visible fragments she scatters throughout the story into a coherent whole. Even if we guess the denouement long before the end, Edith Wharton's employment of dramatic irony means that we still experience a certain frisson as we read Mrs. Ansley's parting shot. As well as being a remarkably useful literary device, dramatic irony gives us a privileged perspective on things, one that allows us to look down with sympathy on the characters of a story and gain a better understanding of the myriad foibles which we, they, and the rest of humanity share.