William Gilpin was one of the 18th century writers who promoted the oval as the most picturesque shape, being that it closely resembled the purview of the human eye. The plates in his numerous books were almost all oval and washed in a continuous tone. Gilpin and others advocated the use of the Claude Glass, a shaped and tinted convex mirror. (Not all Claude Glasses were oval, some were rectangle or circular, but I'd venture to say that the majority were.) The Claude Glass was named in honor of the French painter Claude Lorrain. Further popularized by the poet Thomas Gray with his set viewing "stations" in the English countryside (think Kodak Picture Spots, but with no camera), the Claude Glass was used by British and American tourists from the early 18th century to well into the late 19th century. Oddly, its even shows up in conjunction with tourist photography! I wrote a graduate seminar paper on this device in the late 1990s for Garden History and Picturesque Aesthetics and this MIT book came out in 2004, but there is much more to be done. They rarely pop up on ebay, but I hope to acquire one for myself someday and continue writing/thinking about this fascinating object.
1ST IMAGE (below): Although not an antique Claude Glass and not with black or tinted glass, this image shows how one would be used: a tourist would find a scene to "compose" or "take" and turn their back to admire it! From www.re-picture.info/inspiration/inspiration.html
2ND IMAGE (below): A Claude Glass, Manufactured in England, 18th century
3RD IMAGE (below): The Reverend William Gilpin (1724-1804), View from the bank of a river, watercolour.
4TH IMAGE (below): Julia Margaret Cameron, Mrs. Herbert Duckworth, 1867