What does the word "ficelle" refer to as used in the novel, The Ambassadors?

Ficelle comes from the stage and refers to the tricks and devices now called "special effects." James uses this term as a label for characters whose function assists in firming up the structure of the novel. These characters work much in the same way a letter or other evidence in a detective novel would work; they are an opportunity to fill the reader in on elements otherwise unknown, without using an omniscient narrator or employing interior monologue. Thus, Mrs. Newsome is not a ficelle because she never appears. A ficelle must be present because verbal communication with Strether activates him or her. Miss Gostrey is the purest ficelle in the novel although Waymarsh and Bilham act as ficelles from time to time. When Strether invokes the ficelle, the reader gains information while Strether clarifies his thoughts. The ficelle exists, Strether tells Maria, "to see me through . . . the experience."