From theimperialcourt.tumblr.com/page/9. Also posted to the Bustle Periods - 1870 to 1890 Album here.
While the details of the dresses differ, they are similar in silhouette. The frills suggest the later 1870s or about 1880.
Keywords: Maria Feodorovna (1847-1928), Prinsesse Dagmar, Princess Dagmar, Dagmar (kejserinde af Rusland), Marie Sophie Frederikke Dagmar, Мария Фёдоровна (жена Александра III), Glucksberg family, Romanov family, Princess, Empress, Russian, ribboned cap, chemisette, high enclosing neckline, neck band, pin, fitted jacket bodice, long close sleeves, contrasting cuffs, vee décolletage, revers, bracelets, natural waistline, bustle, over-skirt, close lace-trimmed under-skirt, parasol
Queen Alexandra, Alexandra of Denmark, Alexandra af Danmark, Alexandra Carolina Marie Charlotte Louise Julia, prinsesse af Danmark, Princess of Wales, Queen of the United Kingdom, Glucksberg family, Welf family, Wettin family, Princess, Queen, British, ribboned brim hat, cuirasse bodice, high enclosing neckline, pin, jabot, long tight sleeves, bows, bracelets, natural waistline, close over-skirt, tight flounced under-skirt, shoes