This is one of the four poses known in which the Countess was photographed wearing the "Mathilde" costume. The Countess looks less full-faced here than in her portraits of 1856-7, which suggests that the sitting in Pierson's studio took place several years after Princess Mathilde's ball in February 1857. The costume has been recreated for the photograph, and would then be reinterpreted again by the painter following the directions given by the sitter.
A similar print is in the Montesquiou album (1975.548.82). [PA; "La Divine Comtesse", p. 172]
The "Mathilde" costume is supposedly a dress the Countess wore to a Mardi Gras ball at Princess Mathilde's in Paris, February 27, 1857 (the Princess was Napoleon III's niece); Empress Eugénie had the 18th Century very much in vogue and would wear copies of Marie Antoinette's clothes to such events. [Alteveer/IFA]