http://www.loc.gov/exhibits/empire/
To quote from another post where I got the link from:
"Jaw-droppingly beautiful. The photographer, Sergei Mikhailovich
Prokudin-Gorskii, took 3 separate shots, with deep red, green, and blue
filters, then combined them in slide projections using the same filters.
The Library of Congress, owners of the negatives, used digital
technology (which is similar in concept) to make prints.
"