It's a digital rendering from the original color separation plate. The camera took three filtered exposures in rapid succession on a single plate. The images were displayed by projecting monochrome transparencies with a three-lens magic lantern and filtered light sources.
Details of the process here:
http://www.vintagephoto.tv/expeditions.shtml
and here:
http://www.loc.gov/exhibits/empire/making.html
The Library of Congress is our tax dollars well spent!

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