From Wikimedia; increased exposure.
The given date is 1840, but the dress looks more like 1830 with the ferroniere, natural waistline and off shoulder sweetheart and straight neckline.
Keywords: Rothwell, Mary Wollstonecraft Godwin, Mary Wollstonecraft Shelley, Godwin family, Shelley family, British, straight coiffure, ferroniere, chemise, modesty piece, off shoulder straight neckline, bodice, off shoulder sweetheart neckline, brooch, full sleeves, arm bands, waist band, natural waistline