Princess Alexandra (1844-1925), later Queen Consort of the United Kingdom, dressed as Mary, Queen of Scots for the Waverley Ball in 1871. Apparently the dress still exists and was on display at Buckingham Palace in 2004.
CDV by Oliver Sarony of Scarborough, brother of the more famous Napoleon Sarony of New York.
The same image, on a card by Melhuish, can be found here at another photostream: www.flickr.com/photos/25237851@N03/3519091708/
Fellow Flickrite Turn of the Century II believes that the original photo was taken by the royal photographer Lafayette, with the negatives later being sold to other photographic studios such as those of Melhuish and Sarony.