Photographs

An excellent site 'dedicated to research, dating and preservation of 19th Century photographs' is PhotoTree.com. According to the web site, information found there would be beneficial to ~

Genealogists - Solve genealogy mysteries, learn who that photo is of.
Collectors - Increase your expert knowledge of antique photographs.
Traders - Help establish or confirm the true value of an old image.
Historians - Research & confirm period costume dates.

... and visitors can learn about ~

Daguerreotypes
Ambrotypes
Tintypes
Cartes de Visite
Cabinet Cards
. . . and other formats of the era.